V4L/DVB (3516): Make video_buf more generic

Video_buf were concerned to allow PCI devices to be used as
video capture devices. This patch extends video_buf features
by virtualizing pci-dependent functions and allowing other
type of devices to use it.
It is still DMA centric, although it may be used also by
devices that emulates scatter/gather behavior or a DMA device

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>