(2.10) Introduce hotplug methods (hypervisor inteface)

(2.10) Introduce hotplug methods (hypervisor inteface)

4 new methods: HotAddDevice, HotDelDevice, HotModDevice, HotplugSupported
All these are the hypervisor interface with the backend.
In general they read current runtime info, do the actual hotplug
and update the runtime file.

NIC hot-add:
 - open a tap and get its file descriptor.
 - pass fd with SCM rights (using python-fdsend) via monitor socket
 - create netdev and device with id=kvm_devid and proper pci info

Disk hot-add:
 - create drive with id=kvm_devid
 - create device with id=kvm_devid and corresponding pci info

Use MonitorSocket in _PassTapFd in order to connect to monitor socket
and pass the tap's file descriptor using fdsend.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>