hw/ppc/spapr_pci.c: Avoid functions not in glib 2.12 (g_hash_table_iter_*)

The g_hash_table_iter_* functions for iterating through a hash table
are not present in glib 2.12, which is our current minimum requirement.
Rewrite the code to use g_hash_table_foreach() instead.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>