XenAPI: Cleanup install instructions

* Removed plugin installation through creating a supplemental pack as the
scripts used to build the pack are no longer part of compute.
* Removed references to XCP, Kronos.
* Amend XenServer download links.
* Remove references to DevStack.
* Remove boot from iso instructions as they are not tested.
* Formatted the sources
* Update links to XenServer 6.2

Partial-Bug: 1021224

Change-Id: I02d2906d1d5d697dfe709b4e2cc532f0f89983d9
Merge "XenAPI: Cleanup install instructions"