virtio-input: add multi-touch support

Without multi-touch slots allocated, ABS_MT_SLOT events will be lost by
input_handle_abs_event.

Implementation is based on uinput_create_device.

Signed-off-by: Mathias Crombez <mathias.crombez@faurecia.com>
Co-developed-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Signed-off-by: Vasyl Vavrychuk <vasyl.vavrychuk@opensynergy.com>
Link: https://lore.kernel.org/r/20210115002623.8576-1-vasyl.vavrychuk@opensynergy.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>