tests/vm: run test builds on snapshot

tests/vm: run test builds on snapshot

The build script doesn't shutdown the guest VMs properly,
which results in filesystem corruption and guest boot
failures sooner or later.

Use the --snapshot to run builds on a snapshot,
That way killing the VM doesn't corrupt the base image.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Acked-by: Ed Maste <emaste@freebsd.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190617043858.8290-4-kraxel@redhat.com>
[AJB: added tags]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>