vhost-user: check qemu_chr_fe_set_msgfds() return value

Check qemu_chr_fe_set_msgfds() for errors, to make sure the message to
be sent is correct.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
