KVM: arm64: vgic-its: Maintain a translation cache per ITS

Within the context of a single ITS, it is possible to use an xarray to
cache the device ID & event ID translation to a particular irq
descriptor. Take advantage of this to build a translation cache capable
of fitting all valid translations for a given ITS.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-9-oliver.upton@linux.dev


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