Restore 'tablet mouse on vnc' behavior

We needed to revert commit 5b062a58
because it was in conflict with the usb_mouse hv parameter. Here we
reintroduce its functionality only when usb_mouse is not specified.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>