engine: Limitaion addition to disk extending.

A limitation for extending a disk while it is connected to a paused vm
added to VdcActionUtils which effects both backend and frontend. a disk
cannot be resized while one of the VMs it is connected to is paused.

Change-Id: I2f2c50f2f047bb0c50a7869f4903ad7d0be5290d
Bug-Url: https://bugzilla.redhat.com/1161966
Signed-off-by: Amit Aviram <aaviram@redhat.com>