tests/vm: avoid extra compressed image copy

tests/vm: avoid extra compressed image copy

The image copy is only really needed because xz doesn't know to
properly decompress a file not named properly.  Instead of
decompressing to stdout, and having to rely on a shell, let's just
create a link instead of copying the file.

Signed-off-by: Cleber Rosa <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20190613130718[email protected]>
Signed-off-by: Alex Bennée <[email protected]>