XenAPI: Only parse network bridges if they exist in cmdline

Previously a missing value would return the whole /proc/cmdline
which causes confusion as well as further errors.  The correct
behaviour in this case is to return that it was not specified -
i.e. no value

Change-Id: I90b3e3a4a3947891f4b6887161ef28fe6b400990
Adding support for xenserver local connections

Change-Id: I29ef3d74a30a3b8798c1669bba15205f50ab434e