monitor: fix crash for platforms without a CPU 0

Now that we allow CPU hot unplug on a few platforms, we can end up in a
situation where we don't have a CPU with index 0.  Or at least we could,
if we didn't have code to explicitly prohibit unplug of CPU 0.

Longer term we want to allow CPU 0 unplug, this patch is an early step in
allowing this, by removing an assumption in the monitor code that CPU 0
always exists.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
[dwg: Rewrote commit message to better explain background]
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>