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.