apparmor: avoid denials on libpmem initialization

apparmor: avoid denials on libpmem initialization

With libpmem support compiled into qemu it will trigger the following
denials on every startup.
  apparmor="DENIED" operation="open" name="/"
  apparmor="DENIED" operation="open" name="/sys/bus/nd/devices/"

This is due to [1] that tries to auto-detect if the platform supports
auto flush for all region.

Once we know all the paths that are potentially needed if this feature
is really used we can add them conditionally in virt-aa-helper and labelling
calls in case </pmem> is enabled.

But until then the change here silences the denial warnings seen above.

[1]: https://github.com/pmem/pmdk/blob/master/src/libpmem2/auto_flush_linux.c#L131

Bug: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1871354

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Acked-by: Jamie Strandboge <jamie@canonical.com>