The vgic debug iterator is the final user of vgic_copy_lpi_list(), but
is a bit more complicated to transition to something else. Use a mark
in the LPI xarray to record the indices 'known' to the debug iterator.
Protect against the LPIs from being freed by associating an additional
reference with the xarray mark.

Rework iter_next() to let the xarray walk 'drive' the iteration after
visiting all of the SGIs, PPIs, and SPIs.
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-6-oliver.upton@linux.devSigned-off-by: Marc Zyngier <maz@kernel.org>