HostRebootMixin declares reboot_host(), mixed into DeviceTypeDriver so
any device may be restartable. netOrk restarted hosts by sending
/sbin/reboot through a driver's private _send_command; a driver talking
to an API had no such method and the reboot was silently skipped.
HypervisorDriver gains GUEST_AGENT_PACKAGES / GUEST_AGENT_RUNCMD, the
agent cloud-init installs so the hypervisor can read a new VM's IP.
The default stays qemu-guest-agent; VMware declares open-vm-tools.
NetworkTargetDict.kind may be "portgroup": a VMware port group fixes its
VLAN like an SDN vnet does, without being one.