doc help: Collect block device stuff under its own heading

Collect them from "Standard options", "File system options", "Virtual
File system pass-through options", "Debug/Expert options".
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1360781383-28635-8-git-send-email-armbru@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>