This adds the virt-aa-helper support for gl enabled graphics devices to
generate rules for the needed rendernode paths.

Example in domain xml:
<graphics type='spice'>
  <gl enable='yes' rendernode='/dev/dri/bar'/>
</graphics>

results in:
  "/dev/dri/bar" rw,

Special cases are:
- multiple devices with rendernodes -> all are added
- non explicit rendernodes -> follow recently added virHostGetDRMRenderNode
- rendernode without opengl (in egl-headless for example) -> still add
  the node

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