CA-126097: Handle vm-clone when VBD is attached.

A clone operation was never performed when the VM is running, as a result
there is no need to pause/unpause tapdisk during the clone operation. With
vm-copy in place, there is a change in this behaviour. vm-copy attaches
and activates the VBD, a clone operation during the phase could result
in VHDs in inconsistent state.

Signed-off-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>