hw/usb/bus.c: Handle "no speed matched" case in usb_mask_to_str()

In usb_mask_to_str() we convert a mask of USB speeds into
a human-readable string (like "full+high") for use in
tracing and error messages. However the conversion code
doesn't do anything to the string buffer if the passed in
speedmask doesn't match any of the recognized speeds,
which means that the tracing and error messages will
end up with random garbage in them. This can happen if
we're doing USB device passthrough.

Handle the "unrecognized speed" case by using the
string "unknown".

Fixes: https://bugs.launchpad.net/qemu/+bug/1603785
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20190328133503.6490-1-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/bus.c

@@ -500,6 +500,10 @@ static void usb_mask_to_str(char *dest, size_t size,
                             speeds[i].name);
         }
     }
+
+    if (pos == 0) {
+        snprintf(dest, size, "unknown");
+    }
 }
 
 void usb_check_attach(USBDevice *dev, Error **errp)