Save more of config space for PCI Express and PCI-X devices.

Expand pci_save_state and pci_restore_state to save more of
the config state for PCI Express and PCI-X devices. Various
writable control registers are present in PCI Express that
can potentially be lost over suspend/resume cycle.

This change is modeled after similar functionality in Linux.

Reviewed by: wlosh,jhb
MFC after: 1 month