libs/evtchn: fix build on NetBSD

libs/evtchn: fix build on NetBSD

Use xenio3.h for ioctl definitions

read_exact/write_exact seems to not be available here, which cause a gcc
error.  Use plain read/write, the xenevtchn interface won't do partial
read/write on NetBSD anyway so it should be safe.  This is in line with the
rest of the OS specific helpers.

Fixes: b7f76a699dc ('tools: Refactor /dev/xen/evtchn wrappers into libxenevtchn')
Signed-off-by: Manuel Bouyer <bouyer@netbsd.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>