util: generate a persistent system token

util: generate a persistent system token

When creating the system identity set the system token. The system
token is currently stored in a local path

   /var/run/libvirt/common/system.token

Obviously with only traditional UNIX DAC in effect, this is largely
security through obscurity, if the client is running at the same
privilege level as the daemon. It does, however, reliably distinguish
an unprivileged client from the system daemons.

With a MAC system like SELinux though, or possible use of containers,
access can be further restricted.

A possible future improvement for Linux would be to populate the
kernel keyring with a secret for libvirt daemons to share.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>