Add XenServer to GetDistro's logic

This is primarily to satisfy the expectations of various scripts that os_*
variables are defined, and will result in a distro similar to "xs6.1.0-59235p"

Fixes bug 1211001

Change-Id: I951e1eb3a5e25f4d8773a11b15cf38157b6492fe
Merge "Add XenServer to GetDistro's logic"