PolicyKit configuration for action org.libvirt.unix.manage currently defaults to
auth_self_keep_session, allowing user to authenticate using his/her own password
to use privileged libvirtd services.  This default was picked up to mimic sudo
like behavior.

However, such decision should probably be left to the system administrator and
default policy should be safe, i.e. use auth_admin_keep_session instead. 
Administrator can easily override such default when needed using PolicyKit
configuration files / tools.