core: AuditLogMessage for internal disk addition

When adding a disk as part of an internal command - when no user is
involved, the <UNKNOWN> user was specified as user. This is now
corrected and new audit log messages were added with no user
specification in the message.

Change-Id: Ia66bb5fb393802cbfe6847240d6adfbbd1a53e4e
Bug-Url: https://bugzilla.redhat.com/1149620
Signed-off-by: Vered Volansky <vvolansk@redhat.com>