Input: add new BUS_VIRTUAL bus type

BUS_VIRTUAL can be used when creating virtual devices using uinput driver.

Note that when uinput is used to drive a real piece of hardware "real" bus
type (such as BUS_USB, BUS_BLUETOOTH) should be specified.

Signed-off-by: Michael Hanselmann <linux-kernel@hansmi.ch>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>