block: add support for --image-opts in block I/O tests

Currently all block tests use the traditional syntax for images
just specifying a filename. To support the LUKS driver without
resorting to JSON, the tests need to be able to use the new
--image-opts argument to qemu-img and qemu-io.

This introduces a new env variable IMGOPTSSYNTAX. If this is
set to 'true', then qemu-img/qemu-io should use --image-opts.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1462896689-18450-2-git-send-email-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>