CLOUDSTACK-5834: got VBD statistics from RRD

(cherry picked from commit 0fe1d4bb27de83f3fff4e22d0bb501bc2451e62f)
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

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