hvm/hpet: Only call guest_time_hpet(h) one time per action.

hvm/hpet: Only call guest_time_hpet(h) one time per action.

This call is expensive and will cause extra time to pass.

The software-developers-hpet-spec-1-0a.pdf does not say how long it
takes after the main clock is enabled before the first change of the
master clock.  Therefore multiple calls to guest_time_hpet(h) are
not needed.  Since each timer is started by a loop, each ones start
time will change on the multple calls.  In the real hardware, there
is not delta based on which timer.

Without this change it is possible for an HVM guest running linux to
get the message:

..MP-BIOS bug: 8254 timer not connected to IO-APIC

On the guest console(s); and the guest will panic.

Also Xen hypervisor console with be flooded with:

vioapic.c:352:d1 Unsupported delivery mode 7
vioapic.c:352:d1 Unsupported delivery mode 7
vioapic.c:352:d1 Unsupported delivery mode 7

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>