backup: Implement virsh support for backup

backup: Implement virsh support for backup

Introduce a few more new virsh commands for performing backup jobs.

At this time, I did not opt for a convenience command
'backup-begin-as' that cobbles together appropriate XML from the
user's command line arguments, but that may be a viable future
extension. Similarly, since backup is a potentially long-running
operation, it might be nice to add some sugar that automatically
handles waiting for the job to end, rather than making the user have
to poll or figure out virsh event to do the same.  Eventually, we
will also need a way to create a checkpoint atomically with an
external snapshot.

Signed-off-by: Eric Blake <eblake@redhat.com>