CLOUDSTACK-5834: got VBD statistics from RRD

(cherry picked from commit 4e33d359a82a3708860b24d88b37446cf16e1285)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

Conflicts:
	plugins/hypervisors/xen/src/com/cloud/hypervisor/xen/resource/CitrixResourceBase.java