block/rbd: enable filename option and parsing

When enabling option parsing and blockdev-add for rbd, we removed the
'filename' option.  Unfortunately, this was a bit optimistic, as
previous versions of QEMU allowed the use of the option in backing
filenames via json.  This means that without parsing this option, we
cannot open existing images that used to work fine.

See bug: https://bugzilla.redhat.com/show_bug.cgi?id=1457088

Tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Message-id: 937dc9fde348d13311eb8e23444df3bc3190b612.1497444637.git.jcody@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
