tools/xl: fix segfault in xl migrate --debug

If differences are found during the verification phase of xl migrate
--debug then it is likely to crash with a segfault because the bogus
pagebuf->pfn_types[pfn] is used in a print statement instead of
pfn_type[pfn] .

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>