Add VirtIO devices for improved interactive experience

[edubart]

Context

VirtIO devices can improve the user experience while in interactive mode, by proving better console, shared folders and networking in the virtualized machine:

• Better console
  When running the emulator in interactive mode, it uses a very simple console based on SBI, which has no support for console size and it's not responsive because it doesn't have any concept of interrupts. We could fix those issues and have a better terminal experience by implementing VirtIO console device.

• Shared directories
  We could support sharing directories between the host and guest using VirtIO Plan9 filesystem device while in interactive mode. This would allow to prototype dapps faster, without the need to keep regenerating ext2 filesystems.

• Public networking
  We could support networking to install packages in the guest using VirtIO net device while in interactive mode. This would allow to experiment different packages using the emulator on the fly, while the need to keep maintaning and regenerating a rootfs.

Possible solutions

The PLIC, VirtIO and protocol 9P2000.L specs should be read then the devices implemented accordingly.