Fix GPF when no nics.

kernel/nic.s:
	when no NICs found, [nics] is null - set CF.

	Call class_instanceof instead of DIY.
VirtualBox device updates.

kernel/vbox/vbva.s:
	add graphics adapter driver declarations.

kernel/vbox/vbga.s:
	add guest-additions driver declarations.

kernel/ipiix4.s:
	declare the USB and ACPI driver.

	Remove the dummy classes and reference nulldev for unimplemented code.

kernel/Makefile:
	add VBOX_SRC

kernel/kernel.s:
	include vbox graphics and guest driver code.

kernel/pci.s:
	add constants and declarations for new devices found in VirtualBox.

	Fix dummy-driver instantiation for unknown devices: use nulldev,
	instead of the abstract dev_pci.