virt-aa-helper: clarify command line options

virt-aa-helper: clarify command line options

While only used internally from libvirt the options still are misleading
enough to cause issues every now and then.
Group modes, options and an adding extra file and extend the wording of
the latter which had the biggest lack of clarity.
Both add a file to the end of the rules, but one re-generates the
rules from XML and the other keeps the existing rules as-is not
considering the XML content.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Acked-by: Jamie Strandboge <jamie@canonical.com>
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>