Restructure hypervisor section

This patch splits xen out into intro, configure and install,
in preparation for our documentation restructure.

Existing guides are modified so they aren't adversely affected.
patchset 2 - diane fleming - made editorial changes/spellcheck/formatting

Change-Id: I914e3ab50f0cf55016a5b3b5b81f80cc95cb9eac
bp:restructure-documentation
Merge "Restructure hypervisor section"