virtio-hotplug driver

This driver, if the equvalent guest driver is present in a Linux guest,
allows to programmatically control the CPU online mask of the guest.

The interface to this driver is the virtio_hotplug_do() function call.
test_vhp monitor command to test the virtio_hotplug_do() interface

Before the virtio-hotplug driver can be used by existing code, we can test
that it actually works using this monitor command.