Fix ever increasing CPU usage

In the HostKSM collector we are calculating the number of jiffies that
have been used since the last collection period.  Unfortunately we
never reset self.last_jiff so our count accumulates indefinitely.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=999615

Signed-off-by: Adam Litke <agl@us.ibm.com>
Change-Id: I434dfc454776c8eeb1e2a074132d7b94ca68883f
Reviewed-on: http://gerrit.ovirt.org/18420
Reviewed-by: Martin Sivák <msivak@redhat.com>