savevm: fix wrong initialization by ram_control_load_hook

It should set negative error value rather than 0 in QEMUFile
if there has been an error.

Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Lei Li <lilei@linux.vnet.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
