core: Validate disk registration only if a domain exists.

Registeration of a disk is only a DB operation and does not encounter
any VDS commands, therefore, there should not be any validation for Storage
Domain activation.

Change-Id: I71c138ede22808b5a887e9b9982c154e5591d5c9
Bug-Url: https://bugzilla.redhat.com/1138114
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>