Allow ktr(4) tracing of all guest exceptions via the tunable
"hw.vmm.trace_guest_exceptions". To enable this feature set the tunable
to "1" before loading vmm.ko.

Tracing the guest exceptions can be useful when debugging guest triple faults.

Note that there is a performance impact when exception tracing is enabled
since every exception will now trigger a VM-exit.

Also, handle machine check exceptions that happen during guest execution
by vectoring to the host's machine check handler via "int $18".

Discussed with: grehan
MFC after: 2 weeks