virUSBDeviceNew: Construct vroot path properly

When starting an LXC container, the /dev entries are created
under temp root (/var/run/libvirt/lxc/$name.dev), relabelled and
then the root is pivoted. However, when it comes to USB devices
which keep path to the device in the structure we need a way to
override the default /dev/usb/... path because we want to work
with the one under temp root. That's what @vroot argument is for
in virUSBDeviceNew. However, what is being passed there is:

  vroot = /var/run/libvirt/lxc/lxc_0.dev/bus/usb

Therefore, constructed path is wrong:

  dev->path = //var/run/libvirt/lxc/lxc_0.dev/bus/usb//dev/bus/usb/002/002
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/util/virusb.c

@@ -328,6 +328,7 @@ virUSBDeviceNew(unsigned int bus,
                 const char *vroot)
 {
     virUSBDevicePtr dev;
+    int rc;
 
     if (VIR_ALLOC(dev) < 0)
         return NULL;
@@ -343,9 +344,16 @@ virUSBDeviceNew(unsigned int bus,
         virUSBDeviceFree(dev);
         return NULL;
     }
-    if (virAsprintf(&dev->path, "%s" USB_DEVFS "%03d/%03d",
-                    vroot ? vroot : "",
-                    dev->bus, dev->dev) < 0) {
+
+    if (vroot) {
+        rc = virAsprintf(&dev->path, "%s/%03d/%03d",
+                         vroot, dev->bus, dev->dev);
+    } else {
+        rc = virAsprintf(&dev->path, USB_DEVFS "%03d/%03d",
+                         dev->bus, dev->dev);
+    }
+
+    if (rc < 0) {
         virUSBDeviceFree(dev);
         return NULL;
     }