core: storage domain validation refactoring

Refactored StorageDoaminValidator to return a ValidationResult in all
relevant methods as validators ought to, instead of the old-fashioned
pattern of returning a boolean and accepting a List<String> parameter
for the error messages.

Change-Id: I64734303de8c9cfaaca4fba6bfeeccc93e18c9a6
Signed-off-by: Allon Mureinik <amureini@redhat.com>