qcow2: Add error messages in qcow2_truncate

qemu-img resize has some limitations with qcow2, but the user is only
told that "this image format does not support resize". Quite confusing,
so add some more detailed error_report() calls and change "this image
format" into "this image".

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