For more detail info:
 I will create a VM with vxlan network. The VM will be created failed at process "libvirt.driver._create_domain".
 
the current ovs-info:
[root@localhost ~]# brctl show
bridge name     bridge id               STP enabled     interfaces
virbr0          8000.525400b00efc       yes             virbr0-nic
[root@localhost ~]# 

create VM:
[root@localhost devstack]# nova list
+----+------+--------+------------+-------------+----------+ 
| ID | Name | Status | Task State | Power State | Networks |
+----+------+--------+------------+-------------+----------+
+----+------+--------+------------+-------------+----------+
[root@localhost devstack]# nova  boot --flavor m1.tiny --image 5eff06a2-0078-4894-b150-3681c0a75e96  --nic net-id=9766bbdb-9f6c-42d3-abbe-d948ad95a1f5 vm_test
+--------------------------------------+----------------------------------------------------------------+
| Property                             | Value                                                          |
+--------------------------------------+----------------------------------------------------------------+
| OS-DCF:diskConfig                    | MANUAL                                                         |
| OS-EXT-AZ:availability_zone          |                                                                |
| OS-EXT-SRV-ATTR:host                 | -                                                              |
| OS-EXT-SRV-ATTR:hostname             | vm-test                                                        |
| OS-EXT-SRV-ATTR:hypervisor_hostname  | -                                                              |
| OS-EXT-SRV-ATTR:instance_name        | instance-00000002                                              |
| OS-EXT-SRV-ATTR:kernel_id            | 92f607a2-3992-4b6e-b381-104f42619e53                           |
| OS-EXT-SRV-ATTR:launch_index         | 0                                                              |
| OS-EXT-SRV-ATTR:ramdisk_id           | 7cdd665d-0aed-4bbc-b582-bea9d955f076                           |
| OS-EXT-SRV-ATTR:reservation_id       | r-dgfe3hnw                                                     |
| OS-EXT-SRV-ATTR:root_device_name     | -                                                              |
| OS-EXT-SRV-ATTR:user_data            | -                                                              |
| OS-EXT-STS:power_state               | 0                                                              |
| OS-EXT-STS:task_state                | scheduling                                                     |
| OS-EXT-STS:vm_state                  | building                                                       |
| OS-SRV-USG:launched_at               | -                                                              |
| OS-SRV-USG:terminated_at             | -                                                              |
| accessIPv4                           |                                                                |
| accessIPv6                           |                                                                |
| adminPass                            | uAx2X5SeU3k7                                                   |
| config_drive                         |                                                                |
| created                              | 2016-07-07T14:19:11Z                                           |
| description                          | -                                                              |
| flavor                               | m1.tiny (1)                                                    |
| hostId                               |                                                                |
| host_status                          |                                                                |
| id                                   | 1cb4461e-2457-4b03-be13-e26a141335c4                           |
| image                                | cirros-0.3.4-x86_64-uec (5eff06a2-0078-4894-b150-3681c0a75e96) |
| key_name                             | -                                                              |
| locked                               | False                                                          |
| metadata                             | {}                                                             |
| name                                 | vm_test                                                        |
| os-extended-volumes:volumes_attached | []                                                             |
| progress                             | 0                                                              |
| security_groups                      | default                                                        |
| status                               | BUILD                                                          |
| tags                                 | []                                                             |
| tenant_id                            | 2c5731b1d71b419092220382c50ce2a1                               |
| updated                              | 2016-07-07T14:19:11Z                                           |
| user_id                              | 74fb041674644ddfba0cfdd9ba2d2c95                               |
+--------------------------------------+----------------------------------------------------------------+
[root@localhost devstack]# nova list
+--------------------------------------+---------+--------+------------+-------------+----------+
| ID                                   | Name    | Status | Task State | Power State | Networks |
+--------------------------------------+---------+--------+------------+-------------+----------+
| 1cb4461e-2457-4b03-be13-e26a141335c4 | vm_test | ERROR  | -          | NOSTATE     |          |
+--------------------------------------+---------+--------+------------+-------------+----------+
[root@localhost devstack]# nova delete vm_test
Request to delete server vm_test has been accepted.
[root@localhost devstack]# nova list
+----+------+--------+------------+-------------+----------+
| ID | Name | Status | Task State | Power State | Networks |
+----+------+--------+------------+-------------+----------+
+----+------+--------+------------+-------------+----------+
[root@localhost devstack]# 

Now, do not unplug-vif correctlly,  There are some information was left
[root@localhost ~]# brctl show
bridge name     bridge id               STP enabled     interfaces
qbr2e2fb609-a4          8000.5e21af9a5b6b       no              qvb2e2fb609-a4
                                                        tap2e2fb609-a4
virbr0          8000.525400b00efc       yes             virbr0-nic