win32: use glib gpoll if glib >= 2.50

A fix has been committed in upstream glib commit
210a9796f78eb90f76f1bd6a304e9fea05e97617.
(See also related bug https://bugzilla.gnome.org/show_bug.cgi?id=764415)

It is desirable to use the glib version instead of qemu copy, since it
provides more debugging facilities (G_MAIN_POLL_DEBUG etc), and
hopefully has a better maintainance. Hopefully, we can drop the qemu
copy in a few years.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
