backup: Document new XML for checkpoints

Prepare for new checkpoint APIs by describing the XML that will
represent a checkpoint.  The checkpoint XML is modeled heavily after
virDomainSnapshotPtr. See the docs for more details.

Add testsuite coverage for some minimal uses of the XML (bare minimum,
the sample from html, and a full dumpxml, and some counter-examples
that should fail schema validation). Although use of the REDEFINE flag
will require the <domain> subelement to be present, it is easier for
most of the tests to provide counterpart output produced with the
NO_DOMAIN flag (particularly since synthesizing a valid <domain>
during testing is not trivial).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>