storage: fix scsi detach regression with cgroup ACLs

https://bugzilla.redhat.com/show_bug.cgi?id=876828

Commit 38c4a9cc introduced a regression in hot unplugging of disks
from qemu, where cgroup device ACLs were no longer being revoked
(thankfully not a security hole: cgroup ACLs only prevent open()
of the disk; so reverting the ACL prevents future abuse but doesn't
stop abuse from an fd that was already opened before the ACL change).

Commit 1b2ebf95 overlooked that there were two spots affected.

* src/qemu/qemu_hotplug.c (qemuDomainDetachDiskDevice):
Transfer backing chain before deletion.
* src/qemu/qemu_driver.c (qemuDomainDetachDeviceDiskLive): Fix
spacing (partly to ensure a different-looking patch).

src/qemu/qemu_hotplug.c

@@ -2091,6 +2091,8 @@ int qemuDomainDetachDiskDevice(virQEMUDriverPtr driver,
 
     virDomainDiskRemove(vm->def, i);
 
+    dev->data.disk->backingChain = detach->backingChain;
+    detach->backingChain = NULL;
     virDomainDiskDefFree(detach);
 
     if (virSecurityManagerRestoreImageLabel(driver->securityManager,