io: implement socket watch for win32 using WSAEventSelect+select

On Win32 we cannot directly poll on socket handles. Instead we
create a Win32 event object and associate the socket handle with
the event. When the event signals readyness we then have to
use select to determine which events are ready. Creating Win32
events is moderately heavyweight, so we don't want todo it
every time we create a GSource, so this associates a single
event with a QIOChannel.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>