qemu-img: don't shadow opts variable in img_dd()

It's confusing when two different variables have the same name in one
function.

Cc: Reda Sallahi <fullmanet@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170619150002.3033-1-stefanha@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>