shutdown_host() posts command=shutdown to the node itself; its guests
stop as on any shutdown. begin_stop_vm(name) asks a guest for its clean
shutdown and returns once Proxmox took it, without waiting for the
task: the caller reads get_vm_status until it stopped. Both raise
RuntimeError when Proxmox refuses; an unknown guest is a ValueError.
The contracts HostShutdownMixin and VmStopRequestMixin of
napalm-device-types (NAPALM/napalm-device-types#55); for netOrk's graph
workflows (NetOrk/netork#1636).
