core: Perform hotunplug checks only when dettaching activated disk

When detaching a disk from a vm, the hotplug related checks should be
performed only if the disk is actually plugged to the vm.

Change-Id: Ifc1ac3f30a1f144314cbff51c9c51703d1fb48b1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1029853
Signed-off-by: Liron Aravot <laravot@redhat.com>