rbd: Clean up qemu_rbd_create()'s detour through QemuOpts

The conversion from QDict to QemuOpts is pointless.  Simply get the
stuff straight from the QDict.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1490691368-32099-8-git-send-email-armbru@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>