Fixed: Reset platform_version for XenServer to 5.0, chef-client won't run out-of-the-box cause libc6 is missing

Just tried to install chef through knife bootstrap on some XenServer 6.2 Servers and noticed that Xenserver is treated like RHEL6 because the platform_version is set to 6.2.

Unfortunatly XenServer 6 is based on CentOS 5 and build without libc6, after installation you recieve this error:
/opt/chef/embedded/bin/ruby: /lib/libc.so.6: version `GLIBC_2.6' not found (required by /opt/chef/embedded/lib/libruby.so.1.9)

This PR should fix this behaviour, I hope this is the correct way to contribute. If you want me to add something, drop me a line :D

Cheers,
Jan