monitor: Use QERR_INVALID_PARAMETER instead of QERR_INVALID_CPU_INDEX

This changes the error message from "Invalid CPU index" to "Invalid
parameter index" in the human monitor.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
