core: clear disk device address when it's being unplugged from a vm

Change-Id: If82c8983089916ab1c58dd0330442c3aff8d6d02
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1079697
Signed-off-by: Liron Aravot <laravot@redhat.com>