xen: arm: Implement variable levels in dump_pt_walk

xen: arm: Implement variable levels in dump_pt_walk

This allows us to correctly dump 64-bit hypervisor addresses, which use a 4
level table.

It also paves the way for boot-time selection of the number of levels to use in
the p2m, which is required to support both 40-bit and 48-bit systems.

To support multiple levels it is convenient to recast the page table walk as a
loop over the levels instead of the current open coding.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>