channel-posix: using qemu_set_nonblock() instead of fcntl(O_NONBLOCK)

channel-posix: using qemu_set_nonblock() instead of fcntl(O_NONBLOCK)

Technically, fcntl(soc, F_SETFL, O_NONBLOCK)
is incorrect since it clobbers all other file flags.
We can use F_GETFL to get the current flags, set or
clear the O_NONBLOCK flag, then use F_SETFL to set the flags.

Using the qemu_set_nonblock() wrapper.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Wangxin <wangxinxin.wang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>