x86/hvm/rtc: don't run the vpt timer when !REG_B.PIE

x86/hvm/rtc: don't run the vpt timer when !REG_B.PIE

If the guest has not asked for interrupts, don't run the vpt timer
to generate them.  This is a prerequisite for a patch to simplify how
the vpt interacts with the RTC, and also gets rid of a timer series in
Xen in a case where it's unlikely to be needed.

Instead, calculate the correct value for REG_C.PF whenever REG_C is
read or PIE is enabled.  This allow a guest to poll for the PF bit
while not asking for actual timer interrupts.  Such a guest would no
longer get the benefit of the vpt's timer modes.

Signed-off-by: Tim Deegan <tim@xen.org>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>