KVM: arm64: nv: Turn encoding ranges into discrete XArray stores

In order to be able to store different values for member of an
encoding range, replace xa_store_range() calls with discrete
xa_store() calls and an encoding iterator.

We end-up using a bit more memory, but we gain some flexibility
that we will make use of shortly.

Take this opportunity to tidy up the error handling path.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20240214131827.2856277-11-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>