main-loop: fix qemu_notify_event for aio_notify optimization

aio_notify can be optimized away, and in fact almost always will.  However,
qemu_notify_event is used in places where this is incorrect---most notably,
when handling SIGTERM.  When aio_notify is optimized away, it is possible that
QEMU enters a blocking ppoll immediately afterwards and stays there, without
reaching main_loop_should_exit().

Fix this by using a bottom half.  The bottom half can be optimized too, but
scheduling it is enough for the ppoll not to block.  The hang is thus avoided.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1437738175-23624-1-git-send-email-pbonzini@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
