hypervisor: custom shutdown actions for transient vs persistent VMs

hypervisor: custom shutdown actions for transient vs persistent VMs

It may be desirable to treat transient VMs differently from persistent
VMs. For example, while performing managed save on persistent VMs makes
sense, the same not usually true of transient VMs, since by their
nature they will have no config to restore from.

This also lets us fix a long standing problem with incorrectly
attempting to perform managed save on transient VMs.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>