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).

README.md

@@ -91,8 +91,10 @@ Behaviour shared across roles lives in a function class, exactly once, and a rol
 is a thin bundle over them — `PackageManagementMixin`, `HealthMetricsMixin`,
 `ServiceControlMixin`, `UpdateMixin`, `NatVpnMixin`, `MacAclMixin`, `FirewallRuleMixin`,
 `DhcpServerMixin`, `PingSweepMixin`, `ConfigLifecycleMixin`, `InterfaceFilterMixin`.
-`HostRebootMixin` (`reboot_host`) is mixed into `DeviceTypeDriver` itself, since any
-device may be restartable; like the others it only declares.
+`HostRebootMixin` (`reboot_host`) and `HostShutdownMixin` (`shutdown_host`) are mixed
+into `DeviceTypeDriver` itself, since any device may be restartable or able to shut itself
+down; like the others they only declare. `VmStopRequestMixin` (`begin_stop_vm`) asks for a
+VM's clean shutdown without waiting for it — a hypervisor driver mixes it in when it can.
 
 **Fixes are templates** (`napalm_device_types.fixes`, since 4.13). A change the
 application wants made on a device runs as fixed steps the driver implements, with values