Add CC-BY-SA 3.0 Unported license text

This git repository did not contain the license text directly.

The license was inserted into all the output documents (PDF, HTML, ePub, etc) when the document is formattted using the libvirt-publican brand templates. Specifically the brand defines the CC-BY-SA 3.0 Unported license in Legal_Notice.xml:

https://gitlab.com/libvirt/libvirt-publican/-/blob/master/en-US/Legal_Notice.xml

Make the license of this repo explicit, by copying the full CC-BY-SA 3.0 license text into LICENSE, and the Red Hat clarifying text into COPYING.

Signed-off-by: Daniel P. Berrangé berrange@redhat.com