In order to propagate error message better, convert shpc_init() to
Error also convert the pci_bridge_dev_initfn() to realize.

Cc: mst@redhat.com
Cc: marcel@redhat.com
Cc: armbru@redhat.com
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>