Fix hotplugging of block device-backed usb disks

Commit ca91ba78 moved qemuSetupDiskCgroup into the qemuDomainPrepareDisk
helper, but failed to call it for usb disks.

https://bugzilla.redhat.com/show_bug.cgi?id=1175668`

src/qemu/qemu_hotplug.c

@@ -679,16 +679,8 @@ qemuDomainAttachUSBMassstorageDevice(virConnectPtr conn,
         }
     }
 
-    if (virDomainLockDiskAttach(driver->lockManager, cfg->uri,
-                                vm, disk) < 0)
-        goto cleanup;
-
-    if (virSecurityManagerSetDiskLabel(driver->securityManager,
-                                       vm->def, disk) < 0) {
-        if (virDomainLockDiskDetach(driver->lockManager, vm, disk) < 0)
-            VIR_WARN("Unable to release lock on %s", src);
+    if (qemuDomainPrepareDisk(driver, vm, disk, NULL, false) < 0)
         goto cleanup;
-    }
 
     /* XXX not correct once we allow attaching a USB CDROM */
     if (!src) {
@@ -740,13 +732,7 @@ qemuDomainAttachUSBMassstorageDevice(virConnectPtr conn,
     return ret;
 
  error:
-    if (virSecurityManagerRestoreDiskLabel(driver->securityManager,
-                                           vm->def, disk) < 0)
-        VIR_WARN("Unable to restore security label on %s", src);
-
-    if (virDomainLockDiskDetach(driver->lockManager, vm, disk) < 0)
-        VIR_WARN("Unable to release lock on %s", src);
-
+    ignore_value(qemuDomainPrepareDisk(driver, vm, disk, NULL, true));
     goto cleanup;
 }