New flag bits are worth exposing via virsh.  In the case of
snapshot-list --roots, it's possible to emulate this even when
talking to an older server that lacks the bit; whereas
--metadata requires a newer server.

Although we don't use --security-info yet, the flag is already
documented for other dumpxml operations, and turning it on now
will make it useful when a future patch actually has to honor it.

* tools/virsh.c (cmdSnapshotDumpXML, cmdSnapshotCurrent): Add
--security-info.
(cmdSnapshotList): Add --roots, --metadata.
* tools/virsh.pod (snapshot-dumpxml, snapshot-current)
(snapshot-list): Document these.