core: ImagesHandler: filterImageDisks readability

Improved readability of ImagesHandler.filterImageDisks:
1. Reordered the evaluated conditions
2. Changed the terminology from "filter", which may be ambiguous to
   "allowOnly".

Change-Id: I7cc2fc314e05fda6b1c6cdf39365b64fa0cfe345
Signed-off-by: Allon Mureinik <amureini@redhat.com>
core: ImageHandlerTest - add missing annotation

Added the missing @Test annotation to
testGetAllStorageIdsForImageIds().

Change-Id: I695456e660b7c4debfb3bc1de03b22420d976a8e
Signed-off-by: Allon Mureinik <amureini@redhat.com>