spapr/pci: Correct "does not support hotplugging error messages

When dynamic-reconfiguration is off, hot plug / unplug can fail with
"Bus 'spapr-pci-host-bridge' does not support hotplugging".
spapr-pci-host-bridge is a device, not a bus.  Report the name of the
bus it provides instead: 'pci.0'.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231031111059.3407803-2-armbru@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>