qemu-iothread: IOThread supports the GMainContext event loop

IOThread uses AioContext event loop and does not run a GMainContext.
Therefore,chardev cannot work in IOThread,such as the chardev is
used for colo-compare packets reception.

This patch makes the IOThread run the GMainContext event loop,
chardev and IOThread can work together.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Wang Yong <wang.yong155@zte.com.cn>
Signed-off-by: Wang Guang <wang.guang55@zte.com.cn>
Signed-off-by: Jason Wang <jasowang@redhat.com>