Cleanup of PCI Device initialization:
- handle both IOSpace and 32/64-bit Memory BARs
- add generic function to mm: map(), which maps a physical buffer to a
   newly allocated virtual address range
- add function to memory map the PCI memory bars, warn on multifunction devices
- check pci device header-type to ensure we don't read/write garbage
- remove sys_pci_find_adapter, since it is not used anywhere