Fix Makefile for XenServer builds

The provided Makefile failed to build the agent rpm for XenServer, using
the 6.1 ddk:
- Failed to retrieve version info
- Non unattended yum instruction
- unpackaged files (pyc, pyo) halted the build.

Change-Id: I405aafe5df8a5955cb9f23bcca34037d557bae63