We forget to enable it for transaction .prepare, while it is already
enabled in do_drive_backup since commit a2d665c1
    "blockdev: loosen restrictions on drive-backup source node"

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190618140804.59214-1-vsementsov@virtuozzo.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>