Add new networking infrastructure to Xm-Test. The goal is to make

Add new networking infrastructure to Xm-Test. The goal is to make
creating domains with networking very easy. This patch:

1) Adds new XenDevice class, with the XenNetDevice subclass. These
   classes represent devices for xm-test and are tied to XenDomains.
   This can eventually be used for block devices as well. Currently,
   devices must be added to domains prior to starting the domain. The
   attach and detach needs to be handled.

2) Adds a new NetConfig class to handle configuring the network
   environment in which the tests run. This patch only handles ranges
   of IPs in a bridged environment. DHCP needs to be added as well as
   handling NAT and routed environments.

3) Modifies XenDomain class to handle XenDevices.

4) Adds new configuration options for defining a range of IPs, their
   network address, and their netmask.

5) Removes the old Network.py and Network class.

6) Modifies the existing tests to use the new infrastructure.

7) Adds some documentation to help creating domains.

Signed-off-by: Daniel Stekloff <dsteklof@us.ibm.com>