This patch is to improve the logic of QEMU main thread sleep code in
qemu_chr_write_buffer() where it can be blocked and can't run other
coroutines during COLO IO stress test.

Our approach is to put filter_send() in a coroutine. In this way,
filter_send() will call qemu_coroutine_yield() in qemu_co_sleep_ns(),
so that it can be scheduled out and QEMU main thread has opportunity to
run other tasks.

Signed-off-by: Lei Rao <lei.rao@intel.com>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>