ui: machine: add viommu ComboBox

ui: machine: add viommu ComboBox

Added a proxmoxKVComboBox for selecting a vIOMMU implementation for a VM.
If i440fx is selected, another ComboBox will be enabled/visible that does not
have the Intel option, as Intel-vIOMMU is not compatible with i440fx.

Uses the new machine property-string from the qemu-server's "config: define
machine schema as property-string" commit and the viommu option added in the
qemu-server's "fix #3784: config: Parameter for guest vIOMMU + test-cases"
commit.

Signed-off-by: Markus Frank <m.frank@proxmox.com>