bx_devmodel_c. The early version of the combined vga / svga_cirrus code
required the now obsolete implementation. Some related cleanups in the
devices code.
- Since the pci_read_handler() method is identical in most devices, move it
to the base class to reduce code duplication. Only the 'pcidev' device has
it's own implementation (NOTE: it is not maintained yet).
- Minor other fixes and cleanups in some PCI devices.
