hw/core/loader: gunzip(): initialize z_stream

Coverity signals that variable as being used uninitialized. And really,
when work with external APIs that's better to zero out the structure,
where we set some fields by hand.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Maksim Davydov <davydov-max@yandex-team.ru>
Message-id: 20231017125941.810461-6-vsementsov@yandex-team.ru
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>