HPET should be optional

[Samuel Thibault]

Hello,

As reported on #debian-hurd and https://bugs.debian.org/1132186, using a VM in 
virt-manager leads to 

Crash with "hpet_init: Assertion 'hpet_addr != 0' failed.panic"

We really shouldn't assume that APIC implies HPET.

Samuel