libvirt's backup copies the disks of a running or paused VM from one
instant into new volumes while it keeps running; the clone is defined
shut off as before. The VM is held for the copy, the copy can be
cancelled, and recovery reads the job's records and libvirt.
The clone's definition is one method for both paths; wvmInstance can check,
start, follow, read and abort a push backup of a VM's disks into new
volumes, and freeze/thaw the guest.
