apparmor: fix qemu_bridge_helper for named profile

apparmor: fix qemu_bridge_helper for named profile

Since a3ab6d42 "apparmor: convert libvirtd profile to a named profile"
the detection of the subelement for qemu_bridge_helper is wrong.

In combination with the older 123cc3e1 "apparmor: allow
/usr/lib/qemu/qemu-bridge-helper" it now detects qemu-bridge-helper no
more with its path, but instead as a proper subelement of the named profile
like: label=libvirtd//qemu_bridge_helper

In the same fashion the reverse rule in the qemu_bridge_helper
sub-profile still uses the path and not the named profile label.

Triggering denies like:
apparmor="DENIED" operation="file_inherit"
  profile="libvirtd//qemu_bridge_helper" pid=5629 comm="qemu-bridge-hel"
  family="unix" sock_type="stream" protocol=0 requested_mask="send receive"
  denied_mask="send receive" addr=none peer_addr=none peer="libvirtd"

This patch fixes the unix socket rules for the communication between
libvirtd and qemu-bridge-helper to match that.

Fixes: a3ab6d42d825499af44b8f19f9299e150d9687bc
Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1655111
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>