qemu-img: fix regression copying secrets during convert

When the convert command is creating an output file that needs
secrets, we need to ensure those secrets are passed to both the
blk_new_open and bdrv_create API calls.

This is done by qemu-img extracting all opts matching the name
suffix "key-secret". Unfortunately the code doing this was run after the
call to bdrv_create(), which meant the QemuOpts it was extracting
secrets from was now empty.

Previously this worked by luks as a bug meant the "key-secret"
parameters were not purged from the QemuOpts. This bug was fixed in

  commit b76b4f60
  Author: Kevin Wolf <kwolf@redhat.com>
  Date:   Thu Jan 11 16:18:08 2018 +0100

    qcow2: Use visitor for options in qcow2_create()

Exposing the latent bug in qemu-img. This fix simply moves the copying
of secrets to before the bdrv_create() call.

Cc: qemu-stable@nongnu.org
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>