tests/vm/ubuntu: update i386 image to 18.04

The current image is broken while running qtests but the bug go away
when built with a newer Ubuntu i386 image. I was unable to replicate
the crash on Debian Buster for i386 either so I'm concluding it is a
distro problem. Let's paper over that crack by updating our 32 bir
test image.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>