virt-aa-helper: locking disk files for qemu 2.10

Testing qemu-2.10-rc2 shows issues like:
  qemu-system-x86_64: -drive file=/var/lib/uvtool/libvirt/images/kvmguest- \
  artful-normal.qcow,format=qcow2,if=none,id=drive-virtio-disk0:
  Failed to lock byte 100

It seems the following qemu commit changed the needs for the backing
image rules:

(qemu) commit 244a5668106297378391b768e7288eb157616f64
Author: Fam Zheng <famz@redhat.com>
    file-posix: Add image locking to perm operations

The block appears as:
 apparmor="DENIED" operation="file_lock" [...]
 name="/var/lib/uvtool/libvirt/images/kvmguest-artful-normal.qcow"
 [...] comm="qemu-system-x86" requested_mask="k" denied_mask="k"

With that qemu change in place the rules generated for the image
and backing files need the allowance to also lock (k) the files.

Disks are added via add_file_path and with this fix rules now get
that permission, but no other rules are changed, example:
  -  "/var/lib/uvtool/libvirt/images/kvmguest-artful-normal-a2.qcow" rw,
  +  "/var/lib/uvtool/libvirt/images/kvmguest-artful-normal-a2.qcow" rwk
Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>

src/security/virt-aa-helper.c

@@ -892,11 +892,11 @@ add_file_path(virDomainDiskDefPtr disk,
 
     if (depth == 0) {
         if (disk->src->readonly)
-            ret = vah_add_file(buf, path, "r");
+            ret = vah_add_file(buf, path, "rk");
         else
-            ret = vah_add_file(buf, path, "rw");
+            ret = vah_add_file(buf, path, "rwk");
     } else {
-        ret = vah_add_file(buf, path, "r");
+        ret = vah_add_file(buf, path, "rk");
     }
 
     if (ret != 0)