hw/pci-host/i440fx: Add I440FX_HOST_PROP_PCI_TYPE property

hw/pci-host/i440fx: Add I440FX_HOST_PROP_PCI_TYPE property

I440FX needs a different PCI device model if the "igd-passthru" property is
enabled. The type name is currently passed as a parameter to i440fx_init(). This
parameter will be replaced by a property assignment once i440fx_init() gets
resolved.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230630073720.21297-16-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>