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: Nintrigeri <intrigeri@boum.org>