main-loop: interrupt wait when data arrives on a socket

Right now, the main loop is not interrupted when data arrives on a
socket.  To fix this, register each socket to interrupt the main loop
with WSAEventSelect.  This does not replace select, it only communicates
a change in socket state that requires a select call.

Since the interrupt fires only once per recv call, or only once
after a send call returns EWOULDBLOCK we can activate it on all events
unconditionally.  If QEMU is momentarily uninterested on some condition,
the main loop will not busy wait.  Instead, it may get one extra wakeup,
but then it will ignore the condition until progress occurs and/or
qemu_set_fd_handler is called to set a callback.  At this point the
condition will be tested via select and the callback will be invoked
even if it is still disabled on the event.

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