Add backend method vm_reconfigure on RHEV.

This patch adds the backend method for UI task Reconfigure VM on RHEV.

https://bugzilla.redhat.com/show_bug.cgi?id=1065573

To resolve the above ticket, UI changes are required to enable the task for RHEV VMs and return the result in the format of {"memoryMB"  => total_mem_in_mb, "numCPUs" => num_of_sockets, "numCoresPerSocket" => cores_per_socket}. Please contact me for details about UI changes.