Introduce a bunch of new public APIs related to backup checkpoints.
Checkpoints are modeled heavily after virDomainSnapshotPtr (both
represent a point in time of the guest), although a snapshot exists
with the intent of rolling back to that state, while a checkpoint
exists to make it possible to create an incremental backup at a later
time.  We may have a future hypervisor that can completely manage
checkpoints without libvirt metadata, but the first two planned
hypervisors (qemu and test) both always use libvirt for tracking
metadata relations between checkpoints, so for now, I've deferred
the counterpart of virDomainSnapshotHasMetadata for a separate
API addition at a later date if there is ever a need for it.

Note that until we allow snapshots and checkpoints to exist
simultaneously on the same domain (although the actual prevention of
this will be in a separate patch for the sake of an easier revert down
the road), that it is not possible to branch out to create more than
one checkpoint child to a given parent, although it may become
possible later when we revert to a snapshot that coincides with a
checkpoint.  This also means that for now, the decision of which
checkpoint becomes the parent of a newly created one is the only
checkpoint with no child (so while there are APIs for dealing with a
current snapshot, we do not need those for checkpoints).  We may end
up exposing a notion of a current checkpoint later, but it's easier to
add stuff when proven needed than to blindly support it now and wish
we hadn't exposed it.

The following map shows the API relations to snapshots, with new APIs
on the right:

Operate on a domain object to create/redefine a child:
virDomainSnapshotCreateXML          virDomainCheckpointCreateXML

Operate on a child object for lifetime management:
virDomainSnapshotDelete             virDomainCheckpointDelete
virDomainSnapshotFree               virDomainCheckpointFree
virDomainSnapshotRef                virDomainCheckpointRef

Operate on a child object to learn more about it:
virDomainSnapshotGetXMLDesc         virDomainCheckpointGetXMLDesc
virDomainSnapshotGetConnect         virDomainCheckpointGetConnect
virDomainSnapshotGetDomain          virDomainCheckpointGetDomain
virDomainSnapshotGetName            virDomainCheckpiontGetName
virDomainSnapshotGetParent          virDomainCheckpiontGetParent
virDomainSnapshotHasMetadata        (deferred for later)
virDomainSnapshotIsCurrent          (no counterpart, see note above)

Operate on a domain object to list all children:
virDomainSnapshotNum                (no counterparts, these are the old
virDomainSnapshotListNames           racy interfaces)
virDomainSnapshotListAllSnapshots   virDomainListAllCheckpoints

Operate on a child object to list descendents:
virDomainSnapshotNumChildren        (no counterparts, these are the old
virDomainSnapshotListChildrenNames   racy interfaces)
virDomainSnapshotListAllChildren    virDomainCheckpointListAllChildren

Operate on a domain to locate a particular child:
virDomainSnapshotLookupByName       virDomainCheckpointLookupByName
virDomainSnapshotCurrent            (no counterpart, see note above)
virDomainHasCurrentSnapshot         (no counterpart, old racy interface)

Operate on a snapshot to roll back to earlier state:
virDomainSnapshotRevert             (no counterpart, instead checkpoints
                                     are used in incremental backups via
				     XML to virDomainBackupBegin)
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>