fix #3784: config: Parameter for guest vIOMMU + test-cases

fix #3784: config: Parameter for guest vIOMMU + test-cases

vIOMMU enables the option to passthrough pci devices to L2 VMs in L1
VMs via Nested Virtualisation and adds an extra isolation.

Uses the new property-string from the "config: define machine schema
as property-string"-commit to add the viommu option to the machine
parameter.

Currently there are two vIOMMU implementation in QEMU to choose:
intel or virtio

Virtio-iommu is more recent but less used in production than intel-iommu.

The assert_valid_machine_property function prevents using intel-iommu with
i440fx.

Signed-off-by: Markus Frank <m.frank@proxmox.com>
 [ TL: tiny coding style fix to extract variable inside if expr ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>