Hotplug: rpc support

Hotplug: rpc support

Introduce new RPC that eventually invoke hypervisor specific
hotplug functions. In order to be generic it has the following
arguments: device type, action, device, extra info, seq.
Device type can be NIC or DISK, action can be ADD, REMOVE,
device is the NIC or Disk object, extra info is used by Disk
hotplug to point the device path and seq is the device index
(from the master perspective)

Add HotplugSupported() in all Hypervisors

Only KVM hypervisor supports this method. The other hypervisors
raise HypervisorError.

Before trying hotplug in backend layer invoke hyper.HotplugSupported
and abort RPC in case it is not implemented or for some other reason
not supported (disk+chroot, nic without fdsend, etc).

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