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>

include/kvm/arm_vgic.h

@@ -210,6 +210,12 @@ struct vgic_its {
 	struct mutex		its_lock;
 	struct list_head	device_list;
 	struct list_head	collection_list;
+
+	/*
+	 * Caches the (device_id, event_id) -> vgic_irq translation for
+	 * LPIs that are mapped and enabled.
+	 */
+	struct xarray		translation_cache;
 };
 
 struct vgic_state_iter;