virtio-input: add input routing support

Add display and head properties for input routing to
virtio-input devices, update multiseat documentation.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/multiseat.txt

@@ -2,8 +2,8 @@
 multiseat howto (with some multihead coverage)
 ==============================================
 
-host side
----------
+host devices
+------------
 
 First you must compile qemu with a user interface supporting
 multihead/multiseat and input event routing.  Right now this
@@ -41,6 +41,19 @@ The "display=video2" sets up the input routing.  Any input coming from
 the window which belongs to the video.2 display adapter will be routed
 to these input devices.
 
+Starting with qemu 2.4 and linux kernel 4.1 you can also use virtio
+for the input devices, using this ...
+
+	-device pci-bridge,addr=12.0,chassis_nr=2,id=head.2 \
+	-device secondary-vga,bus=head.2,addr=02.0,id=video.2 \
+	-device virtio-keyboard-pci,bus=head.2,addr=03.0,display=video.2 \
+	-device virtio-tablet-pci,bus=head.2,addr=03.0,display=video.2
+
+... instead of xhci and usb hid devices.
+
+host ui
+-------
+
 The sdl2 ui will start up with two windows, one for each display
 device.  The gtk ui will start with a single window and each display
 in a separate tab.  You can either simply switch tabs to switch heads,