core,webadmin: Added flag on storage pool that marks a local fs pool

As a part of the effort to allow mixed domains pool, the checks on
a storage pool to check if it's local will now be with a designated method
isLocal() which return true if the pool is local

Relates-To: https://bugzilla.redhat.com/1038053
Change-Id: I4f1067fd1d299a93b9555c4714b4e85ff980a830
Signed-off-by: Tal Nisan <tnisan@redhat.com>