pseries: Clean up error reporting in htab migration functions

The functions for migrating the hash page table on pseries machine type
(htab_save_setup() and htab_load()) can report some errors with an
explicit fprintf() before returning an appropriate error code.  Change some
of these to use error_report() instead. htab_save_setup() is omitted for
now to avoid conflicts with some other in-progress work.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>