XenAPI: Support local connections

Currently there is no way to connect to XAPI through the unix socket,
requiring a URL and credentials.

This change allows you to run in dom0 (particularly in xenserver-core
environments) and connect to XAPI using the unix domain socket by
specifying "unix://local"
DocImpact

Change-Id: I3d5ace31c9c0411fa711cfdf87383d36f61bfa4a
Merge "XenAPI: Support local connections"