infra/common: document KVM Unit Testing

Add a new document to explain the procedure to validate kernel KVM
and the userspace hypervisor program kvmtool using the standard set
of unit testcases from kvm-unit-tests project.

Signed-off-by: Sathyam Panda <sathyam.panda@arm.com>
Change-Id: I9fa63e66272c86ea5936c40bb27d29421c4fd80e