main-loop: replace WaitForMultipleObjects with g_poll

On w32, glib implements g_poll using WaitForMultipleObjects
or MsgWaitForMultipleObjects.  This means that we can simplify
our code by switching to g_poll, and at the same time prepare for
adding back glib sources.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
