iohandler.c shares the same interface with aio, but with duplicated
code. It's better to rebase iohandler, also because that aio is a
more friendly interface to multi-threads.

Create a global AioContext instance and let its GSource handle the
iohandler events.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1441596538-4412-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>