usb: update docs for bus name change

At some point the default usb bus name changed from 'usb.0' to
'usb-bus.0' (probably as part of the qom conversion).  Update
the usb documentation accordingly.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

docs/usb2.txt

@@ -11,7 +11,7 @@ one USB 2.0 bus driven by the EHCI controller.  Devices must be
 attached to the correct controller manually.
 
 The '-usb' switch will make qemu create the UHCI controller as part of
-the PIIX3 chipset.  The USB 1.1 bus will carry the name "usb.0".
+the PIIX3 chipset.  The USB 1.1 bus will carry the name "usb-bus.0".
 
 You can use the standard -device switch to add a EHCI controller to
 your virtual machine.  It is strongly recommended to specify an ID for
@@ -27,7 +27,7 @@ a complete example:
         -drive if=none,id=usbstick,file=/path/to/image   \
         -usb                                             \
         -device usb-ehci,id=ehci                         \
-        -device usb-tablet,bus=usb.0                     \
+        -device usb-tablet,bus=usb-bus.0                 \
         -device usb-storage,bus=ehci.0,drive=usbstick
 
 This attaches a usb tablet to the UHCI adapter and a usb mass storage
@@ -88,22 +88,22 @@ ports (1-4), the emulated (1.1) USB hub has eight ports.
 
 Plugging a tablet into UHCI port 1 works like this:
 
-        -device usb-tablet,bus=usb.0,port=1
+        -device usb-tablet,bus=usb-bus.0,port=1
 
 Plugging a hub into UHCI port 2 works like this:
 
-        -device usb-hub,bus=usb.0,port=2
+        -device usb-hub,bus=usb-bus.0,port=2
 
 Plugging a virtual usb stick into port 4 of the hub just plugged works
 this way:
 
-        -device usb-storage,bus=usb.0,port=2.4,drive=...
+        -device usb-storage,bus=usb-bus.0,port=2.4,drive=...
 
 You can do basically the same in the monitor using the device_add
 command.  If you want to unplug devices too you should specify some
 unique id which you can use to refer to the device ...
 
-        (qemu) device_add usb-tablet,bus=usb.0,port=1,id=my-tablet
+        (qemu) device_add usb-tablet,bus=usb-bus.0,port=1,id=my-tablet
         (qemu) device_del my-tablet
 
 ... when unplugging it with device_del.
@@ -148,10 +148,10 @@ using for testing is bus 1 + port 1 for 2.0 devices and bus 3 + port 1
 for 1.1 devices.  Passing through any device plugged into that port
 and also assign them to the correct bus can be done this way:
 
-    qemu -M pc ${otheroptions}                           \
-        -usb                                             \
-        -device usb-ehci,id=ehci                         \
-        -device usb-host,bus=usb.0,hostbus=3,hostport=1  \
+    qemu -M pc ${otheroptions}                               \
+        -usb                                                 \
+        -device usb-ehci,id=ehci                             \
+        -device usb-host,bus=usb-bus.0,hostbus=3,hostport=1  \
         -device usb-host,bus=ehci.0,hostbus=1,hostport=1
 
 enjoy,