Add support for multiple virtio net devices

[andreeaflorescu]

Once we integrate the vm-allocator (#132) we will be able to use more than one block & net device respectively. For this to work, we also need to make updates to the kernel command line on x86_64 to add multiple devices, and to the FDT on arm. Also, we need to update the virtio code for the devices because right now at least the block device works with the assumption that there is a single block on /dev/vda. The code that needs to be updated is here: https://github.com/rust-vmm/vmm-reference/tree/main/src/devices/src/virtio

We should add a python tests to check that these devices work. For network device we should make sure that both interfaces work.