Add support for virt machine (AArch64)

[willdurand]

Although this is going to be yet another board to maintain, and this one does not correspond to any real hardware, it would be interesting to have a virt machine for AArch64 because:

• that would allow to verify that the arch code works on two different boards
• that would allow to test the network stack (virtio-net driver needed) on a different target, with a different driver

See: https://qemu.readthedocs.io/en/latest/system/arm/virt.html