vmd(8): protect global vm and vcpu state with mutex.

vmd(8): protect global vm and vcpu state with mutex.

The vm process uses multiple pthreads to emulate vcpu and also drive
an event loop doing various io emulation. At points, one or the
other needs to read/write global vm state bits and toggle if a vcpu
is halted or "done."

This adds in a another mutex to protected the global state and
untangles areas where the mutex for protecting a condition variable
was being used around modifying some global state.

ok mlarkin@