infra/common: document virtual interrupts supported in virtualization

Add a new document that introduces to the various type of virtual
interrupts supported in KVM assisted by Neoverse hardware design
Platform. Starting with feature of direct injection of vLPI and vSGI.
This document explains the procedure to test these feature using
platform Software. This document guides to build software stack,
resolve prerequisites and setup the required test environment.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: I517b787441c0a08b446e3fc299b7606495d90732