qed: support for growing images

The .bdrv_truncate() operation resizes images and growing is easy to
implement in QED.  Simply check that the new size is valid and then
update the image_size header field to reflect the new size.

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