sampling.ImagePathStatus: drop unused code, and only it

The ImagePathStatus is never started, but one of its methods is used by
HostStatsThread.get(). This patch moves the still-used bits of
ImagePathStatus code into HostStatsThread, and drops the unused
ImagePathStatus thread.

Change-Id: I530b6d83c6ab5d0cc52df797ae3e40998fb77435
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21038