nvdimm memory is backed by a path on the host. This currently works only via
hotplug where the AppArmor label is created via the domain label callbacks.

This adds the virt-aa-helper support for nvdimm memory devices to generate
rules for the needed paths from the initial guest definition as well.

Example in domain xml:
  <memory model='nvdimm'>
    <source>
      <path>/tmp/nvdimm-base</path>
    </source>
    <target>
     <size unit='KiB'>524288</size>
     <node>0</node>
    </target>
  </memory>
Works to start now and creates:
  "/tmp/nvdimm-base" rw,

Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1757085Acked-by: NJamie Strandboge <jamie@canonical.com>
Signed-off-by: NChristian Ehrhardt <christian.ehrhardt@canonical.com>