APIC access virtualization

[asamy]

Seems to generate #DF for some reason on setup. Maybe the APIC access page is bad.

[asamy]

47a5a28 fixes this but the exit-handlers need to be adjusted to make this work, so the controls are all commented out for now. Broadwell seems to support virtualizing APIC accesses, posted interrupts and TPR shadow, but the other features are not available (register virtualization, x2apic, etc.). Posted interrupts is really cool especially for nesting (ack intr on exit, etc.).

See: vcpu_handle_apic_access, vcpu_handle_apic_write in exit.c