xenapi: define LIBVIRT_{TYPE,GROUP}

Define some values for LIBVIRT_TYPE and LIBVIRT_GROUP for the xenserver
case.

Fixes bug 1184960

Change-Id: Ie68d7695eb0ab2eed8967005bb9a85139a420eec
xenapi - quantum: L2 agents for dom0 and domU

This patch makes it easier to support the all-in-one installation of
XenServer with Quantum:
- separate agents for domU and dom0 openvswitches. The domU agent's name
  is "q-domua", and the dom0 agent is called "q-agt"
- create a physnet1 physical network to connect those openvswitches

Note: This patch depends on the quantum patch:

    https://review.openstack.org/15022

For a diagram, see:
    http://goo.gl/BuAdg

Change-Id: Iff851438553528b9051edd380ecbb2c0415cbf78