qmp: Disable query-cpu-* commands when they're unavailable

Instead of requiring clients to actually call the query-cpu-*
commands to find out if they are implemented, remove them from
the output of "query-commands", so clients know they are not
available.

This is implemented by extending the existing hack at
qmp_unregister_commands_hack(). I wish I could avoid adding even
more #ifdefs to that code, but that's the solution we have today.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <1475696941-8056-1-git-send-email-ehabkost@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
