hw/riscv/virt.c: add virtio-iommu-pci hotplug support

hw/riscv/virt.c: add virtio-iommu-pci hotplug support

We want to add a RISC-V 'virt' libqos machine to increase our test
coverage. Some of the tests will try to plug a virtio-iommu-pci
device into the board and do some tests with it.

Enable virtio-iommu-pci in the 'virt' machine.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240217192607.32565-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>