hvm: Timer fixes:

hvm: Timer fixes:
 1. Do not record more than one pending interrupt in
 no-missed-tick-accounting mode. We do not stack up missed interrupts
 in this timer mode.
 2. Always record all missed ticks when we are in a
 missed-tick-accounting mode. Do not have a ceiling for this as it
 simply causes guests to lose track of wall time.
 3. General bits of cleanup and simplification.
From: Dave Winchell <dwinchell@virtualiron.com>
Signed-off-by: Keir Fraser <keir@xensource.com>