src: introduce helper API for creating GSource for socket

src: introduce helper API for creating GSource for socket

We need to be able to create event loop watches using the
GSource API for sockets. GIOChannel is able todo this, but
we don't want to use the GIOChannel APIs for reading/writing,
and testing shows just using its GSource APIs is unreliable
on Windows.

This patch thus creates a standalone helper API for creating
a GSource for a socket file descriptor. This impl is derived
from code in QEMU's io/channel-watch.c file that was written
by myself & Paolo Bonzini & thus under Red Hat copyright.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>