Tweak debug log messages

Tweak debug log messages

- Turn tracing messages into DPRINTF (only compiled with DEBUG).

- Pass __func__ to vm_stop and vm_remove: this way we can track who
called the function in the async context.  It replaces the manual
log_debug in front of each vm_stop/vm_remove.  This debug logging
trick can be removed in the future once we are more confident about
it.

OK ccardenas@ mlarkin@