virttest: Add libvirt polkit config support and testing

This is for libvirt API ACL testing.

2 commits included:
virttest.virsh: Add support of unprivileged virsh session
virttest: Libvirt polkit config support in env_process

The libvirtd and polkit rule setup require run as root, after env setup done, use su or ssh login to unprivileged user shell then start virsh which connect to qemu:///system (default will be qemu:///session). 

The change will not affect current cases as user need set setup_libvirt_polkit = 'yes' and specify polkit rule args in cfg file to enable ACL testing.

A simple ACL testing case will be added in tp-libvirt later.

Reference:
http://libvirt.org/acl.html#perms