core: filter candidate block Storage Domains with existing LUNs.

Filter out candidates of imported Storage Domain if we have external
LUNs which are part of the vg.

Change-Id: Id82e5e1d3df8ce265833efb3fc4b9050ca73b679
Bug-Url: https://bugzilla.redhat.com/1157243
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>