vmm: drop misleading iommu option from serial config

`--serial` still accepted `iommu=on|off` from the shared serial/console
configuration shape, even though only virtio-console has a meaningful
IOMMU connection.

Cloud Hypervisor wires virtio-iommu support through DMA-capable
virtio/VFIO PCI endpoints. The serial devices are legacy UARTs accessed
through PIO or MMIO registers, so exposing an IOMMU option there is
misleading.

On-behalf-of: Philipp Schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

fuzz/fuzz_targets/http_api.rs

@@ -177,7 +177,6 @@ impl RequestHandler for StubApiRequestHandler {
                         mode: ConsoleOutputMode::Tty,
                         socket: None,
                     },
-                    iommu: false,
                 },
                 console: ConsoleConfig {
                     common: CommonConsoleConfig {