Add a new VCPU conf option, that allows userland to request VMEXITs after a
TPR change. This is supported on all Intel CPUs, and not-too-old AMD CPUs.

The reason for wanting this option is that certain OSes (like Win10 64bit)
manage interrupt priority in hardware via CR8 directly, and for these OSes,
the emulator may want to sync its internal TPR state on each change.

Add two new fields in cap.arch, to report the conf capabilities. Report TPR
only on Intel for now, not AMD, because I don't have a recent AMD CPU on
which to test.

(maxv)