use polkit authorization

[OndrejHome]

Document how to use polkit in libvirt for minimal operation of fast-vm. This should allow the tighter restrictions for libvirt to fast-vm users useful in shared environments.

https://libvirt.org/aclpolkit.html

Idea:

• make example of minimal policy that can create and operate VMs with fast-vm prefix and keep everything else disallowed
• make example of more relaxed policy where all operations are permitted for the fast-vm group so this can be used as in-place replacement for the current setting (in 1.2) that changes the permissions on libvirt socket to allow this group - ideally make this work without need to change default libvirtd.conf