qemu-img: add support for --object with 'dd' command

The qemu-img dd command added --image-opts support, but missed
the corresponding --object support. This prevented passing
secrets (eg auth passwords) needed by certain disk images.

Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170515164712.6643-2-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>