qemu-iotests: Check autodel behaviour for device_del

Block devices creates with -drive and drive_add should automatically
disappear if the guest device is unplugged. blockdev-add ones shouldn't.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
