Add a test case for converting an empty image (which only returns zeroes
when read) to a preallocated encrypted qcow2 image.
qcow2_has_zero_init() should return 0 then, thus forcing qemu-img
convert to create zero clusters.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20190724171239.8764-10-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>