[XEN] Truncate arithmetic for PERFSTATUS counters to the width of perfc_t.

[XEN] Truncate arithmetic for PERFSTATUS counters to the width of perfc_t.
Since these are all calculated per-cpu now, a status counter that is
incremented on one cpu and decremented on another will underflow on
the decrement.  When we add the per-cpu values to get the total, if
we don't truncate the sum it will be max-perfc + 1 instead of zero.
Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>