[HVM] Clean up two small APIC TDCR issues.

[HVM] Clean up two small APIC TDCR issues.

First, only save the bits that can be set, so that on a subsequent
read can't see MBZ bits as set. Next, be sure to initialize the
timer_divide_count, which is derived from the TDCR.  This avoids a
potential divide-by-zero elsewhere in the code.

The original patch was provided by Ben Thomas <ben@virtualiron.com>.
This modified version refactors the code slightly and renames
timer_divide_count to timer_divisor.

Signed-off-by: Keir Fraser <keir@xensource.com>