Block driver documentation is available in qemu-doc.html.  It would be
convenient to have documentation for formats, protocols, and filter
drivers in a man page.

Extract the relevant part of qemu-doc.html into a new file called
docs/qemu-block-drivers.texi.  This file can also be built as a
stand-alone document (man, html, etc).
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>