core: Avoid NPE when changing image status (868887)

Sometimes BaseImagesCommand.setImageStatus() checked for
getRelevantDiskImage() without checking for null. Higher up in the
hierarchy the getDiskImage returned value is tested for null (before unlockImage()
calls), even though the status
change is always done on getRelevantDiskImage() returned value.
Now what is tested for null before unlockImage() is
getRelevantDiskImage().

Change-Id: Ica20a2542c07a355720503c25a14fbae3394c4d4
Bug-Url: https://bugzilla.redhat.com/868887
Signed-off-by: Vered Volansky <vvolansk@redhat.com>