The virtual machine is saved in the image format (ceph rbd backend) as qcow2 instead of raw

[weisongf]

* OpenStack version:Rocky

* The virtual machine is saved in the image format (ceph rbd backend) as qcow2 instead of raw.

* Operation process：

1.openstack server image create --name ubuntu-1804-2nics-arm64 ubuntu18.04-arm
2.ubuntu18.04-arm :Virtual machines booted from a volume

* openstack image show ubuntu-1804-2nics-arm64

| Field | Value |

| checksum | d41d8cd98f00b204e9800998ecf8427e |
| container_format | bare |
| created_at | 2020-02-18T07:08:56Z |
| disk_format | qcow2 |
| file | /v2/images/426fa33f-db87-4092-be20-1d664ad8cb6b/file |
| id | 426fa33f-db87-4092-be20-1d664ad8cb6b |
| min_disk | 40 |
| min_ram | 0 |
| name | ubuntu-1804-2nics-arm64 |
| owner | 5d51ef16f6644af2971560618c93f071 |
| properties | base_image_ref='1efee0bd-d53e-4e57-b069-30a554e5c523', bdm_v2='True', block_device_mapping='[{"guest_format": null, "boot_index": 0, "delete_on_termination": true, "no_device": null, "snapshot_id": "34c5c105-5430-45c4-903c-08538086084d", "device_name": "/dev/sda", "disk_bus": "scsi", "image_id": null, "source_type": "snapshot", "tag": null, "device_type": "disk", "volume_id": null, "destination_type": "volume", "volume_size": 40}]', boot_roles='reader,member,admin', direct_url='rbd://e7787af0-aa37-42a7-8fc6-a4a1320866e8/glance.images/426fa33f-db87-4092-be20-1d664ad8cb6b/snap', hw_disk_bus='scsi', hw_firmware_type='uefi', hw_qemu_guest_agent='yes', hw_scsi_model='virtio-scsi', image_location='image', image_state='available', image_type='image', instance_uuid='f5eda779-9446-4f62-8165-62ab342c7511', locations='[{u'url': u'rbd://e7787af0-aa37-42a7-8fc6-a4a1320866e8/glance.images/426fa33f-db87-4092-be20-1d664ad8cb6b/snap', u'metadata': {}}]', os_hash_algo='sha512', os_hash_value='cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e', os_hidden='False', owner_id='5d51ef16f6644af2971560618c93f071', owner_project_name='admin', owner_user_name='admin', root_device_name='/dev/sda', user_id='b4653e7431264ebf99809dc8efe9e49b' |
| protected | False |
| schema | /v2/schemas/image |
| size | 0 |
| status | active |
| tags | |
| updated_at | 2020-02-18T07:39:32Z |
| virtual_size | None |
| visibility | public