hw/pci: disable pci-bridge's shpc by default

hw/pci: disable pci-bridge's shpc by default

The shpc component is optional while  ACPI hotplug is used
for hot-plugging PCI devices into a PCI-PCI bridge.
Disabling the shpc by default will make slot 0 usable at boot time
and not only for hot-plug, without loosing any functionality.
Older machines will have shpc enabled for compatibility reasons.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>