backup: test: Implement metadata tracking for checkpoint APIs
backup: test: Implement metadata tracking for checkpoint APIs

A lot of this work heavily copies from the existing snapshot APIs.
The test driver doesn't really have to do anything more than just
expose an interface into libvirt metadata, making it possible to test
saving and restoring XML, and tracking relations between multiple
checkpoints.

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

crobinso/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,andreabolognani/libvirt,jardasgit/libvirt,crobinso/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,crobinso/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,jardasgit/libvirt,jfehlig/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,libvirt/libvirt,olafhering/libvirt