core: AddStorageDomainCommand.getTargetStoragePool

Extracted AddStorageDomainCommand.getTargetStoragePool for later use
(not to mention the increased readability).

Change-Id: I3776beb5a7426e010f05ca0b4f3dffb128346ed7
Related-To: https://bugzilla.redhat.com/1120712
Signed-off-by: Allon Mureinik <amureini@redhat.com>