feat: shutdown_host and begin_stop_vm, shutting down without waiting

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).

CHANGELOG.md

@@ -7,6 +7,14 @@ recorded here; their history is the git log.
 
 ## [Unreleased]
 
+### Added
+- `HostShutdownMixin.shutdown_host()`, on `DeviceTypeDriver`: shut the device itself
+  down cleanly through its own means; declared, so `hasattr` says whether a driver
+  can. No forced variant (NetOrk/netork#1636).
+- `VmStopRequestMixin.begin_stop_vm(name)`: ask for a VM's clean shutdown and return
+  at once, for callers that wait on a timer instead of in `stop_vm`. No role demands
+  it (NetOrk/netork#1636).
+
 ## [4.22.0] - 2026-10-23
 
 ### Added