chardev: replace qemu_set_nonblock()

Those calls are either for non-socket fd, or are POSIX-specific. Use the
dedicated GLib API. (qemu_set_nonblock() is for socket-like)

(this is a preliminary patch before renaming qemu_set_nonblock())

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>