tests/vm: proper guest shutdown

tests/vm: proper guest shutdown

When not running in snapshot mode ask the guest to poweroff and wait for
this to finish instead of simply quitting qemu, so the guest can flush
pending updates to disk.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190617043858.8290-5-kraxel@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>