Change error reporting from return value to Error argument.

Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
[AF: Fixed cpu_get_memory_mapping() documentation]
Signed-off-by: Andreas Färber <afaerber@suse.de>