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
xenapi: kernel_cmdline moved out from template

The kernel's cmdline was set during the initial devstack machine
installation. Thus, during second runs, the kernel's cmdline was not
updated. This patch extracts append_kernel_cmdline, and configures
domU's kernel cmdline every time. As some networking parameters are
passed through the kernel cmdline, this patch makes it possible to
change the network configuration, even if a cached devstack exists.

Related to blueprint xenapi-devstack-cleanup

Change-Id: I3b7175f4e83326c3e28825ac50625f6bd2a9a029