Temporary volume could be deleted with force

[Hironori Shiina]

A temporary volume is created when an attached volume is backed up. This temporary volume can be deleted by DELETE API with `force`.

This is a similar issue to bug 1970768[1], which was fixed. There is still a possibility of the similar issue because `force delete` ignores volume status.

[1] https://bugs.launchpad.net/cinder/+bug/1970768

Steps to reproduce:
1. Create a volume:
   $ openstack volume create --size=20 myvolume
2. Attach the volume to a server:
   $ openstack server add volume myvolume myserver --device /dev/vdb
3. Backup the volume:
   $ openstack volume backup create myvolume --force
4. List volumes during backing up and find a temporary volume:
   $ openstack volume list
   +--------------------------------------+-------------------------------------------------+------------+------+---------------------------------------------------------------+
   | ID | Name | Status | Size | Attached to |
   +--------------------------------------+-------------------------------------------------+------------+------+---------------------------------------------------------------+
   | 287b0cee-4ca2-41d4-8e98-d6d705751851 | backup-vol-c146c1fc-864d-4269-b369-dc5bfffca42f | available | 20 | |
   | c146c1fc-864d-4269-b369-dc5bfffca42f | myvolume | backing-up | 20 | Attached to 78ce3358-364d-4500-b122-4bbef72b99cd on /dev/vdb |
   +--------------------------------------+-------------------------------------------------+------------+------+---------------------------------------------------------------+
5. Delete the temporary volume with the `--force` option:
   $ openstack volume delete --force backup-vol-c146c1fc-864d-4269-b369-dc5bfffca42f

[Sofia Enriquez (lsofia-enriquez)]

Fix proposed to master https://review.opendev.org/c/openstack/cinder/+/830901