HostStats: recalculate total line rate every time

This should have been done when we started supporting host plugging in
commit b7b29e6.

Change-Id: Iad7f7eb1922967e2e8174b2b2a25f06fbf4c7d50
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29469
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>