apparmor: add rules to use qemu_bridge_helper

apparmor: add rules to use qemu_bridge_helper

Due to mediation of socket and signal activity currently qemu:///session
connections calling qemu_bridge_helper fail.

We need the profile for libvirtd itself and the subprofile for
qemu-bridge-helper to be able to talk/notify to each other via unix socket and
signals.

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1754871
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>