The PCI-DMA-mapping.txt moved to general docs and became DMA-API-HOWTO.txt
in 5e07c2c7 ("Documentation: rename PCI/PCI-DMA-mapping.txt to
DMA-API-HOWTO.txt").  Add new file about PCI Express I/O Virtualization.
Signed-off-by: NErik Ekman <erik@kryo.se>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>