input: virtio: don't send mouse wheel event twice

On Linux, a mouse event is generated for both down and up when mouse
wheel is used. This caused virtio_input_send() to be called twice each
time the wheel was used.

This commit adds a check for the button down state and only calls
virtio_input_send() when it is true.

Signed-off-by: Miika S <miika9764@gmail.com>
Message-Id: <20171222152531.1849-4-miika9764@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>