Add XenServer to grains and set proper virtual pkg

Add XenServer to _OS_FAMILY and set usage of yumpk5 for XenServer 6 and below.  This is very similar to #4277.

Relavent grains from XenServer 6 (after this patch):
 'os': 'XenServer',
 'os_family': 'RedHat',
 'oscodename': 'xenenterprise',
 'osfullname': 'XenServer',
 'osrelease': '6.0.0',
 'virtual': 'xen',
 'virtual_subtype': 'Xen Dom0'