hw/pci-bridge: Convert pxb initialization functions to Error

Firstly, convert pxb_dev_init_common() to Error and rename
it to pxb_dev_realize_common().
Actually, pxb_register_bus() is converted as well.

And then,
convert pxb_dev_initfn() and pxb_pcie_dev_initfn() to Error,
rename them to pxb_dev_realize() and pxb_pcie_dev_realize()
respectively.

Signed-off-by: Wei Jiangang <weijg.fnst@cn.fujitsu.com>
Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>