Introduce a bunch of new virsh commands for managing checkpoints in
isolation. More commands are needed for performing incremental
backups, but these commands were easy to implement by modeling heavily
after virsh-snapshot.c. There is no need for checkpoint-revert or
checkpoint-current since those snapshot APIs have no checkpoint
counterpart.  Similarly, it is not necessary to change which
checkpoint is current when redefining from XML, since until we
integrate checkpoints with snapshots, there is only a linear chain
(and you can deduce the current checkpoint by instead using
'checkpoint-list --leaves').  Other aspects of checkpoint-list are
also a bit simpler than the snapshot counterpart, in part because we
don't have to cater to back-compat to older API.

Upcoming patches will test these interfaces once the test driver
supports checkpoints.
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>