hw/i386/intel_iommu: Fix out-of-bounds access on guest IRT

vtd_irte_get failed to check the index against the configured table
size, causing an out-of-bounds access on guest memory and potentially
misinterpreting the result.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Message-Id: <4b15b728-bdfe-3bbe-3a5c-ca3baeef3c5c@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>