apparmor: pass attach_disconnected

to cure

 + virsh lxc-enter-namespace --noseclabel sl /bin/ls /bin/ls
 libvirt:  error : Expected at least one file descriptor
 error: internal error: Child process (2714) unexpected exit status 125

caused by

 apparmor="DENIED" operation="open" info="Failed name lookup - disconnected path" error=-13 profile="/usr/sbin/libvirtd" name="" pid=1422 comm="libvirtd" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

examples/apparmor/usr.sbin.libvirtd

@@ -2,7 +2,7 @@
 #include <tunables/global>
 @{LIBVIRT}="libvirt"
 
-/usr/sbin/libvirtd {
+/usr/sbin/libvirtd flags=(attach_disconnected) {
   #include <abstractions/base>
   #include <abstractions/dbus>