x86: extend debug key 't' to collect useful clock skew info

x86: extend debug key 't' to collect useful clock skew info

By repeating "xm debug-key t; xm dmesg | tail -N" in
dom0 (e.g. with the "watch" command), useful skew info
about the machine can be collected, which will be very
helpful for diagnosing customer problems on misbehaving
machines.

Patch details:
1) Min and max readings from the current sample are useless
so get rid of those but retain the current maxdif (stime skew).
2) Remember max of skew, sum of skews, and a count
so average skew can be displayed.
3) Also record info for cycles (tsc on x86, itc on ia64).

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>