APIC hardware assistance selection

Description:

ABI that got committed in [cset] doesn't describe the properties to construct the VM properly. We need at least one extra bit that matches what hardware can do and lets us be safe.

Issue is the hardware acceleration development: it wasn't on/off, but bit by bit. Lack of a TPR bit caused errors in what "accelerated APIC" meant.

Once this gets in, we need to change Jan's patch that adds this support for PVH. (Unlinked here to not introduce a back-link.)