core: Show warning when kdump integration required but not configured

Show warning when updating host configuration if PM and kdump
integration are enabled, but VDSM doesn't report kdump status as
enabled.

Change-Id: I1c9a89daa1a106206bf581bb0f996bb84c203cfa
Bug-Url: https://bugzilla.redhat.com/1126854
Signed-off-by: Martin Perina <mperina@redhat.com>