hpet: Add MSI support

hpet: Add MSI support

This implements the HPET capability of routing IRQs to the front-side
bus, aka MSI support. This feature can be enabled via the qdev property
"msi" and is off by default.

Note that switching it on can cause guests (at least Linux) to use the
HPET as timer instead of the LAPIC. KVM users should recall that only
the latter is currently available as fast in-kernel model.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>