ui: input-linux: Add absolute event support

ui: input-linux: Add absolute event support

This patch adds support for absolute pointer events to the input-linux
subsystem. This support was omitted from the original input-linux patch,
however most of the code required for it is already in place.

Support for absolute events is especially useful for guests with vga
passthrough. Since they have a physical monitor, none of normal channels
for sending video output (vnc, etc) are used, meaning they also can't be
used to send absolute input events. This leaves QMP as the only option
to send absolute input into vga passthrough guests, which is not its
intended use and is not efficient.

This patch allows, for example, uinput to be used to create virtual
absolute input devices. This lets you build external systems which share
physical input devices between guests. Without absolute input
capability, such external systems can't seamlessly share pointer devices
between guests.

Signed-off-by: Philippe Voinov <philippevoinov@gmail.com>
Message-id: 20170505134231.30210-1-philippevoinov@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>