iotests: Convert to preallocated encrypted qcow2

iotests: Convert to preallocated encrypted qcow2

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 <[email protected]>
Acked-by: Stefano Garzarella <[email protected]>
Tested-by: Stefano Garzarella <[email protected]>
Message-id: 20190724171239[email protected]
Reviewed-by: Maxim Levitsky <[email protected]>
Signed-off-by: Max Reitz <[email protected]>