core: Add method to determine if StorageDomainType is of type ISO or Export

The checks if a storage domain is of ISO/Export or Data/Master repeats itself
throughout the code base, there is already a method on the StorageDomainType
enum to determine if the type is Data/Master, added another method to
determine if the type is ISO/Export and replaced the checks for each combination
with the respected method

Change-Id: I8d10d0eca9127b3e0e6fbf89484301e61bf233b2
Signed-off-by: Tal Nisan <tnisan@redhat.com>