KVM: selftests: Add quadword MMIO accessors

The base registers in the GIC ITS and redistributor for LPIs are 64 bits
wide. Add quadword accessors to poke at them.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-16-oliver.upton@linux.dev


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