rumpuser_exit(), _exit(): Cleanly halt Mini-OS.

rumpuser_exit() and _exit() were calling minios_do_exit() which is
intended to be called from an exception context and/or other "panic"
situation.  This was causing the stack trace code in minios_do_exit() to
walk off into never never land when the rumprun-xen application called
exit() or returned from main().

This commit adds a new minios_do_halt(reason) function, with the reason
code intended to mirror SHUTDOWN_* in xen/sched.h. Of those, currently
only poweroff and crash are implemented.

We then modify the rumpuser_exit() and _exit() functions to correctly
shut down the domain by calling minios_stop_kernel() followed by
minios_do_halt(MINIOS_HALT_POWEROFF).

Signed-off-by: Martin Lucina <martin@lucina.net>