Enable the "Acknowledge Interrupt on VM exit" VM-exit control.

This control is needed to enable "Posted Interrupts" and is present in all
the Intel VT-x implementations supported by bhyve so enable it as the default.

With this VM-exit control enabled the processor will acknowledge the APIC and
store the vector number in the "VM-Exit Interruption Information" field. We
now call the interrupt handler "by hand" through the IDT entry associated
with the vector.