Since commit cb4e5f8e, monitor command change makes the new media
readonly iff the type hint is BDRV_TYPE_CDROM, i.e. the drive was
created with media=cdrom.  The intention is to avoid changing a block
device's read-only-ness.  However, BDRV_TYPE_CDROM is only a hint.  It
is currently sufficent for read-only.  But it's not necessary, and it
may not remain sufficient.

Use bdrv_is_read_only() instead.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>