XCP <= 1.6 should use yumpkg5

Configure yumpkg5 as the virtual package for pkg on XCP <= 1.6.  Per http://wiki.xen.org/wiki/Xen_/_XCP_/_XCP_on_Linux_Overview, Xen 1.6 uses a CentOS 5 base.  Additional cases for selection of yumpkg vs. yumpkg5 may need to be added in the future.