qemu-iotests: Test reopen between read-only and read-write

This serves as a regression test for the bugs that were just fixed for
bdrv_reopen() between read-only and read-write mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>