xen: silence messages

Xen boot is not totally silent. The devices usually print through
device_printf, which right now is hooked at printf. Changing that to debugf
already gets rid of most messages. However, netfront by itself manually prints
some messages with printf. They are changed as well.

Please note that the messages in netfront are changed directly to debug,
instead to device_printfs as well. This is because although we just hook that
to a printer, device_printf is supposed to prepend the messages with device
information.  We don't do it now, but could do it in the future. That is why
the original code used printf directly, and I am keeping that intent.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>