core: LUNs validation on storage creation

On create/extend storage domain, validate that the specified
LUNs aren't already part of another storage domain
or used by DirectLUN disks.

Change-Id: Ia39a19374169fce20850b187a6d4201ff34ef0d8
Bug-Url: https://bugzilla.redhat.com/955661
Signed-off-by: Daniel Erez <derez@redhat.com>