Invalid root device name for volume-backed instances with libvirt

[Feodor Tersin]

Since that https://review.openstack.org/#/c/189632/ is merged, root device name of volume backed instances is /dev/vdb with libvirt.

Steps to reproduce against DevStack:
1 Boot an instance:
nova --block-device source=image,dest=volume,size=1,bootindex=0,id=<xxx> --flavor m1.nano inst
where xxx is an image id.

2 Look at the device name:
openstack volume list

Expected value: /dev/vda
Actual value: /dev/vdb

Inside guest OS the volume is displayed as /dev/vda.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/208796

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/208796
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=0ed3f33028b539877abf51b5a36a0f8e5c0a5927
Submitter: Jenkins
Branch: master

commit 0ed3f33028b539877abf51b5a36a0f8e5c0a5927
Author: Feodor Tersin <email address hidden>
Date: Mon Aug 3 21:12:38 2015 +0300

    libvirt: Fix root device name for volume-backed instances

    Since that I76a7cfd995db6c04f7af48ff8c9acdd55750ed76 was merged, root
    device name is assigned /dev/vdb for volume-backed instances with
    libvirt.

    The reasons are:
    1 now device names are reset to None for all bdms before libvirt's
    get_disk_mapping call;
    2 get_disk_mapping processes the root bdm by get_info_from_bdm twice:
    as a root bdm, and as a regular bdm;
    3 for each call of get_info_from_bdm the root bdm is passed with no
    device name;
    4 get_info_from_bdm generates a new device name for each call with empty
    device name.

    So vda is assigned for 'root' record in mapping, but vdb - for the
    volume record.

    This patch updates root bdm after the first call, thus the second call
    is performed with certain device name.

    Closes-Bug: #1481164
    Change-Id: I3ce12563846b2c34ac778d44e7582eef930ae4b0