doc: Document driver-specific -blockdev options

This documents the driver-specific options for the raw, qcow2 and file
block drivers for the man page. For everything else, we refer to the
QAPI documentation.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>