KVM: selftests: Add a minimal library for interacting with an ITS

A prerequisite of testing LPI injection performance is of course
instantiating an ITS for the guest. Add a small library for creating an
ITS and interacting with it from the guest.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-17-oliver.upton@linux.dev


Signed-off-by: Marc Zyngier <maz@kernel.org>