Expose the recent API additions in virsh.  Borrows ideas from 'dominfo'
for the general type of information to display.

Output looks like:

$ tools/virsh snapshot-info fedora-local tmp
Name:           tmp
Domain:         fedora-local
Current:        no
State:          disk-snapshot
Parent:         -
Children:       1
Descendants:    2
Metadata:       yes

possibly with fewer lines when talking to older servers.

* tools/virsh.c (cmdSnapshotInfo): New command.
* tools/virsh.pod (snapshot-info): Document it.