vm: always log when guest CPU state changes

In order to improve debuggability of VDSM,
add a log when the guest CPU state changes.

Since this event is supposed to be sporadic,
this will not add noise to the logs.

Change-Id: Ia0749c5082954b7c9ed8781a860d7e538d34aa7e
Relates-To: https://bugzilla.redhat.com/1131548
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31697
Reviewed-by: Dan Kenigsberg <danken@redhat.com>