KVM security improvements

This MR will do the following things for the KVM SR:

• validate the parameters from the JSON
• add basic auth on nginx frontend (also move the noVNC socket to a unix socket (instead of local IPv4 + port) and remove the password)
• add an option to totally disable the noVNC frontend

This MR needs:

• https://lab.nexedi.com/nexedi/slapos.toolbox/-/merge_requests/162