x86/HPET: cache MSI message last written

x86/HPET: cache MSI message last written

Rather than spending measurable amounts of time reading back the most
recently written message, cache it in space previously unused, and thus
accelerate the CPU's entering of the intended C-state.

hpet_msi_read() ends up being unused after this change, but rather than
removing the function, it's being marked "unused" in order - that way
it can easily get used again should a new need for it arise.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>