HostShutdownMixin.shutdown_host(), on DeviceTypeDriver: a device shuts
itself down cleanly through its own means; declared, so hasattr says
whether a driver can. There is no forced variant.
VmStopRequestMixin.begin_stop_vm(name): ask for a VM's clean shutdown
and return at once, for a caller that waits on a timer and reads
get_vm_status instead of blocking in stop_vm. No role demands it.
For netOrk's graph workflows, which switch a hypervisor off when CI has
nothing for it (NetOrk/netork#1636).
christianmanivong