xenapi: Use separate agents for dom0 and domU

This patch makes it easier to support the all-in-one installation of
XenServer:
- separate separate agents for domU and dom0 openvswitches
- create a physnet1 physical network

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

Change-Id: Iff851438553528b9051edd380ecbb2c0415cbf78