docs: add qemu-block-drivers(7) man page

docs: add qemu-block-drivers(7) man page

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: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>