Fix reset of cgroup when detaching USB device from LXC guests

When detaching a USB device from an LXC guest we must remove
the device from the cgroup ACL. Unfortunately we were telling
the cgroup code to use the guest /dev path, not the host /dev
path, and the guest device node had already been unlinked.
This was, however, fortunate since the code passed &priv->cgroup
instead of priv->cgroup, so would have crash if the device node
were accessible.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 2c2bec94)

src/lxc/lxc_driver.c

@@ -3870,7 +3870,7 @@ lxcDomainDetachDeviceHostdevUSBLive(virLXCDriverPtr driver,
     }
 
     if (!(usb = virUSBDeviceNew(def->source.subsys.u.usb.bus,
-                                def->source.subsys.u.usb.device, vroot)))
+                                def->source.subsys.u.usb.device, NULL)))
         goto cleanup;
 
     VIR_DEBUG("Unlinking %s", dst);
@@ -3884,7 +3884,7 @@ lxcDomainDetachDeviceHostdevUSBLive(virLXCDriverPtr driver,
 
     if (virUSBDeviceFileIterate(usb,
                                 virLXCTeardownHostUsbDeviceCgroup,
-                                &priv->cgroup) < 0)
+                                priv->cgroup) < 0)
         VIR_WARN("cannot deny device %s for domain %s",
                  dst, vm->def->name);