blockdev: Clean up how readonly persists across virtual media change

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>

blockdev.c

@@ -589,7 +589,7 @@ int do_change_block(Monitor *mon, const char *device,
     if (eject_device(mon, bs, 0) < 0) {
         return -1;
     }
-    bdrv_flags = bdrv_get_type_hint(bs) == BDRV_TYPE_CDROM ? 0 : BDRV_O_RDWR;
+    bdrv_flags = bdrv_is_read_only(bs) ? 0 : BDRV_O_RDWR;
     if (bdrv_open(bs, filename, bdrv_flags, drv) < 0) {
         qerror_report(QERR_OPEN_FILE_FAILED, filename);
         return -1;