virtio_input: Prevent EV_MSC/MSC_TIMESTAMP loop storm for MT.

In 'commit 29cc309d

 ("HID: hid-multitouch: forward MSC_TIMESTAMP")',
EV_MSC/MSC_TIMESTAMP is added to each before EV_SYN event. EV_MSC is
configured as INPUT_PASS_TO_ALL.
In case of a touch device which report MSC_TIMESTAMP:
  BE pass EV_MSC/MSC_TIMESTAMP to FE on receiving event from evdev.
  FE pass EV_MSC/MSC_TIMESTAMP back to BE.
  BE writes EV_MSC/MSC_TIMESTAMP to evdev due to INPUT_PASS_TO_ALL.
  BE receives extra EV_MSC/MSC_TIMESTAMP and pass to FE.
>>> Each new frame becomes larger and larger.

Disable EV_MSC/MSC_TIMESTAMP forwarding for MT.

V2: Rebase.

Signed-off-by: Colin Xu <colin.xu@intel.com>
Link: https://lore.kernel.org/r/20210202001923.6227-1-colin.xu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>