engine: Use isBlockDomain() where possible

Replaced all the places that checked if a StorageType was ISCSI or FCP
with isBlockDomain().

This achieves two purposes:
1. The code is cleaner, and easier to read.
2. It protects against possible future bugs if a third block storage
   type is added.

Change-Id: I60fc4cd333e954cc5075f3b3040054a7213a2f71
Signed-off-by: Allon Mureinik <amureini@redhat.com>
core: ImagesHandler.CheckImageConfiguration format

Improved the formatting of ImagesHandler.CheckImageConfiguration in
order to make it easier to read:
1. Removed useless parentheses that don't add readability
2. Reformatted so each major condition (separated by ||) starts in its
   own new line.

Change-Id: I07e4622ce2b6a6f90d88f72e37cdca0df313399b
Signed-off-by: Allon Mureinik <amureini@redhat.com>