hw/i386/intel_iommu: vtd_slpte_nonzero_rsvd(): assert no overflow

We support only 3- and 4-level page-tables, which is firstly checked in
vtd_decide_config(), then setup in vtd_init(). Than level fields are
checked by vtd_is_level_supported().

So here we can't have level out from 1..4 inclusive range. Let's assert
it. That also explains Coverity that we are not going to overflow the
array.

CID: 1487158, 1487186
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: 20231017125941.810461-2-vsementsov@yandex-team.ru
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>