hw/pcie: Introduce a base class for PCI Express Root Ports

hw/pcie: Introduce a base class for PCI Express Root Ports

The 'base' PCI Express Root Port includes
the common code to be re-used for all
Root Ports implementations. Most of the code
was taken from the current implementation
of Intel's IOH 3420 Root Port.

Signed-off-by: Marcel Apfelbaum <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>