tests/vm: move images to $HOME/.cache/qemu-vm/images

tests/vm: move images to $HOME/.cache/qemu-vm/images

It's easier to move around the images then, by replacing the
subdirectory with a symlink.  Allows to share the images between
multiple qemu checkouts for example.

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