udev: expose USB 3.0 devices for PV USB

Even if particular PV USB implementation doesn't support it, still have
it included in QubesDB. It should be up to attaching code to decide.

Also, don't fail if xen-usbback module doesn't exist. This isn't the
only option (the other one is usbip over qrexec).

QubesOS/qubes-issues#531

udev/udev-usb-add-change

@@ -24,8 +24,10 @@ DESC="${ID_VENDOR_ID}:${ID_MODEL_ID} ${ID_SERIAL}"
 VERSION=`cat /sys/$DEVPATH/version`
 if [ "${VERSION}" = " 1.00" -o "${VERSION}" = " 1.10" ] ; then
 	VERSION=1
-elif [ "${VERSION}" = " 2.00" ] ; then
+elif [ "${VERSION}" = " 2.00" -o "${VERSION}" = " 2.10" ] ; then
 	VERSION=2
+elif [ "${VERSION}" = " 3.00" -o "${VERSION}" = " 3.10" ] ; then
+	VERSION=3
 else
 	# FIXME: silently ignoring devices with unexpected USB version
 	exit 0
@@ -38,4 +40,4 @@ qubesdb-write "$QDB_KEY/usb-ver" "$VERSION"
 qubesdb-write /qubes-usb-devices ''
 
 # Make sure PVUSB backend driver is loaded.
-/sbin/modprobe xen-usbback 2> /dev/null || /sbin/modprobe usbbk
+/sbin/modprobe xen-usbback 2> /dev/null || true