[all variants] Why VirtualBox can bridge NIC without root and libvirt qemu:///session can not?

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http://ubuntuforums.org – Hi everybody, do you know how can I bridge a guest network interface with the host one with libvirt in qemu:///session without root privilege? I'm working in depth on this problem, and I can't figure out a way to use something different from NAT (interface type="user") for my qemu VMs in session mode (with not-root user). I know that VirtualBox can bridge Guest interfaces with the host one without root privilege! I wonder if this can be possible for libvirt qemu:///session too. (Hardware)