build-kvmtool: add a build script for kvmtool and KVM unit testcases

Add a script to build, clean and package kvmtool which is used to setup
Linux KVM based virtual machines on a Linux host OS. Also add provision
to build, clean and package kvm-unit-tests which is used to carry out
unit testing for Linux KVM hypervisor.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: I71e7dfaa2af9f35297acf23af2c74987ddc0e1c7