util: add note about event file descriptors on Windows

util: add note about event file descriptors on Windows

When using GNULIB with Winsock, libvirt will never see the normal HANDLE
objects, instead GNULIB guarantees that libvirt gets a C runtime file
descriptor. The GNULIB poll impl also expects to get C runtime file
descriptors rather than HANDLE objects. Document this behaviour so that
it is clear to applications providing event loop implementations if they
need Windows portability.

Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>