qemu uses wheel-up/down button events for mouse wheel input, however
linux applications typically want REL_WHEEL events.

This fixes wheel with linux guests. Tested with X11/wayland, and
windows virtio-input driver.

Based on a patch from Marc.
Added property to enable/disable wheel axis.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170926113243.26081-1-kraxel@redhat.com