AppArmor: allow virt-aa-helper read access to Nova's qcow backing files.

As reported on https://bugs.debian.org/892431, without this rule, when launching
a QEMU KVM instance, an error occurs immediately upon launching the QEMU
process such as:

Could not open backing file: Could not open
'/var/lib/nova/instances/_base/affe96668a4c64ef380ff1c71b4caec17039080e':
Permission denied

The other instance disk images are already covered by the existing rule:

/**/disk{,.*} r

Signed-off-by: intrigeri <intrigeri@boum.org>