tests: add migrate-during-backup

tests: add migrate-during-backup

Add a simple test which tries to run migration during backup.
bdrv_inactivate_all() should fail. But due to bug (see next commit with
fix) it doesn't, nodes are inactivated and continued backup crashes
on assertion "assert(!(bs->open_flags & BDRV_O_INACTIVE));" in
bdrv_co_write_req_prepare().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210911120027.8063-2-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>