First cut of new time interfaces and synchronisation mechanisms.

First cut of new time interfaces and synchronisation mechanisms.
Based on an initial patch from Don Fry at IBM.
Still TODO:
 1. Testing
 2. NTP synchronisation
 3. Fix wallclock interface a bit
 4. Support for platform timers other than PIT (e.g., HPET, IBM Cyclone)
 5. Scale 64-bit TSC diffs instead of 32-bit, just for sanity
 6. Error-correcting scale factor is still slightly wrong
 6. More testing
Signed-off-by: Keir Fraser <keir@xensource.com>