x86/hvm: avoid p2m lookups for vlapic accesses.

x86/hvm: avoid p2m lookups for vlapic accesses.

The LAPIC base address is a known GFN, so we can skip looking up the
p2m: we know it should be handled as emulated MMIO.  That helps
performance in older Windows OSes, which make a _lot_ of TPR accesses.

This will change the behaviour of any OS that maps other
memory/devices at its LAPIC address; the new behaviour (the LAPIC
mapping always wins) is closer to actual hardware behaviour.

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