x86/VPMU: handle APIC_LVTPC accesses

x86/VPMU: handle APIC_LVTPC accesses

Don't have the hypervisor update APIC_LVTPC when _it_ thinks the vector
should be updated. Instead, handle guest's APIC_LVTPC accesses and write what
the guest explicitly wanted (but only when VPMU is enabled).

This is updated version of commit 8097616fbdda that was reverted by
cc3404093c85. Unlike the previous version, we don't update APIC_LVTPC
when VPMU is disabled to avoid interfering with NMI watchdog (which
runs only when VPMU is off).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>