init_blk_migration_it() skips drives with type hint BDRV_TYPE_CDROM.
The intention is to skip read-only drives.  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: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>