since there is no user of qemu_char_get_next_serial any more,
it's time to let it go away.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: xiaoqiang zhao <zxq_yx_007@163.com>
Message-id: 1465028065-5855-7-git-send-email-zxq_yx_007@163.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>