vmspawn: Disable hpet for vmspawn x86 virtual machines

vmspawn: Disable hpet for vmspawn x86 virtual machines

hpet is an emulated clocksource that is generally discouraged in favor
of kvm-clock or tsc for virtual machines. While vmspawn's virtual machines
already use kvm-clock, leaving hpet enabled causes qemu on the host to
consume a non-trivial amount of cpu, so let's disable the hpet feature since
we're not making use of it anyway.