(2.10) Check if hotplug is supported in CheckPrereq

(2.10) Check if hotplug is supported in CheckPrereq

Introduce new RPC hotplug_supported that invokes the corresponding
hypervisor's method which checks if hotplug is generally
supported. Call this RPC early in CheckPrereq() and abort if
hotplug is not supported. Currently only KVM hypervisor with
qemu versions > 1.0. Please note that this check is unaware of
target device and hotplug action. This latter check is still
done during hotplug_device() RPC.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

Conflicts:
	lib/cmdlib/instance.py
	test/py/cmdlib/instance_unittest.py

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>