core: Fix NPE on ChangeCD with 'Down' VM

Moved invocation of ImagesHandler.cdPathWindowsToLinux after check that
assures vm is running and storage domain available.

Change-Id: Ie3bf10dbeb8dd645a515dc140bd9081dc1d1acab
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1067407
Signed-off-by: Martin Betak <mbetak@redhat.com>