docs: updated qemu-img man page and qemu-doc to reflect VHDX support.

The man page for qemu-img, and the qemu-doc, did not mention VHDX
as a supported format.  This adds in reference to VHDX in those
documents.

[Stefan Weil <sw@weilnetz.de> suggested s/Block Size/Block size/ for
consistency.  I have made this change.
--Stefan]

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>