apparmor: allow unix stream for p2p migrations

On live migration with --p2p like:
 $ virsh migrate --live --p2p kvmguest-bionic-normal \
       qemu+ssh://10.6.221.80/system

We hit an apparmor deny like:
  apparmor="DENIED" operation="file_inherit"
  profile="/usr/sbin/libvirtd" pid=23477 comm="ssh" family="unix"
  sock_type="stream" protocol=0 requested_mask="send receive"
  denied_mask="send" addr=none peer_addr=none peer="unconfined"

The rule is not perfect, but can't be restricted further at the moment
(new upstream kernel features needed). For now the lack of a profile on the
peer as well as comm not being a conditional on rules do not allow to filter
further.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>

examples/apparmor/usr.sbin.libvirtd

@@ -53,6 +53,9 @@
   network packet dgram,
   network packet raw,
 
+  # for --p2p migrations
+  unix (send, receive) type=stream addr=none peer=(label=unconfined addr=none),
+
   ptrace (trace) peer=unconfined,
   ptrace (trace) peer=/usr/sbin/libvirtd,
   ptrace (trace) peer=/usr/sbin/dnsmasq,