block: add bdrv_set_read_only() helper function

We have a helper wrapper for checking for the BDS read_only flag,
add a helper wrapper to set the read_only flag as well.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 9b18972d05f5fa2ac16c014f0af98d680553048d.1491597120.git.jcody@redhat.com
