KVM: arm64: vgic-v3: Iterate the xarray to find pending LPIs

KVM: arm64: vgic-v3: Iterate the xarray to find pending LPIs

Start walking the LPI xarray to find pending LPIs in preparation for
the removal of the LPI linked-list. Note that the 'basic' iterator
is chosen here as each iteration needs to drop the xarray read lock
(RCU) as reads/writes to guest memory can potentially block.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240221054253.3848076-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>