vmm(4): add dt tracepoints for guest entry/exit

vmm(4): add dt tracepoints for guest entry/exit

To aid in development and debugging, this adds a tracepoint prior
to vm entry and after vm exit. It captures the vcpu and run params
plus the exit code, but dt(4)/btrace(8) will need some future work
to leverage those args.

The location of the tracepoint might change in the future, but for
now this solves my issues trying to use printf's to debug vmcs state
corruption.

ok mpi@