Fix -usbdevice crash

If -usbdevice is used on a machine with no USB busses, usb_create
will fail and return NULL.  Patch below handles this failure gracefully
rather than crashing when we try to init the device.
Signed-off-by: NPaul Brook <paul@codesourcery.com>

hw/usb-bus.c

@@ -102,6 +102,9 @@ USBDevice *usb_create(USBBus *bus, const char *name)
 USBDevice *usb_create_simple(USBBus *bus, const char *name)
 {
     USBDevice *dev = usb_create(bus, name);
+    if (!dev) {
+        hw_error("Failed to create USB device '%s'\n", name);
+    }
     qdev_init_nofail(&dev->qdev);
     return dev;
 }

hw/usb-msd.c

@@ -592,6 +592,9 @@ static USBDevice *usb_msd_init(const char *filename)
 
     /* create guest device */
     dev = usb_create(NULL /* FIXME */, "usb-storage");
+    if (!dev) {
+        return NULL;
+    }
     qdev_prop_set_drive(&dev->qdev, "drive", dinfo);
     if (qdev_init(&dev->qdev) < 0)
         return NULL;

hw/usb-net.c

@@ -1491,6 +1491,9 @@ static USBDevice *usb_net_init(const char *cmdline)
     }
 
     dev = usb_create(NULL /* FIXME */, "usb-net");
+    if (!dev) {
+        return NULL;
+    }
     qdev_set_nic_properties(&dev->qdev, &nd_table[idx]);
     qdev_init_nofail(&dev->qdev);
     return dev;

hw/usb-serial.c

@@ -594,6 +594,9 @@ static USBDevice *usb_serial_init(const char *filename)
         return NULL;
 
     dev = usb_create(NULL /* FIXME */, "usb-serial");
+    if (!dev) {
+        return NULL;
+    }
     qdev_prop_set_chr(&dev->qdev, "chardev", cdrv);
     if (vendorid)
         qdev_prop_set_uint16(&dev->qdev, "vendorid", vendorid);