hw/pci: Have safer pcie_bus_realize() by checking error path

hw/pci: Have safer pcie_bus_realize() by checking error path

While pci_bus_realize() currently does not use the Error* argument,
it would be an error to leave pcie_bus_realize() setting bus->flags
if pci_bus_realize() had failed.

Fix by using a local Error* and return early (propagating the error)
if pci_bus_realize() failed.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210201153700.618946-1-philmd@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>