Currently, VirtualPDU uses 'libvirtd' group, which errors since this
group is not used anymore on ubuntu because we now use the same group
that debian uses 'libvirt'.
This commit change the used group to 'libvirt'. This is the right group
and it's allowed to do changes on virsh.
I would rather use LIBVIRT_GROUP however a bug in the selection logic
sets it to 'libvirtd' group as soon as it exists, even if both exists.
Change-Id: Iddb61356b8a92184ca60a078ea9d929e5b51e629