Add support for mounting a shared folder between the host and guest via virtiofs (v2)

[nathanchance]

virtiofs, available in QEMU 5.2 or newer and Linux guests 5.4 or newer, is a more modern way to pass local folders along to QEMU, as it takes advantage of the fact that the folders are on the same machine as the hypervisor.

To use virtiofs, we first need to find and run virtiofsd, which has two different implementations: a C implementation included with QEMU up until 8.0 (available on most distros) and a standalone Rust implementation available on GitLab (not packaged on many distros but easy to build and install). Once we find it, we run it in the background and connect to it using some QEMU parameters, which were shamelessly taken from the official virtiofs website:

https://virtio-fs.gitlab.io/howto-qemu.html

To use it within the guest (you can use a different path than /mnt/shared but mount -t virtio shared must be used):

  # mkdir /mnt/shared
  # mount -t virtiofs shared /mnt/shared
  # echo "$(uname -a)" >/mnt/shared/foo

On the host:

  $ cat shared/foo
  Linux (none) 6.1.0-rc8-next-20221207 #2 SMP PREEMPT Wed Dec  7 14:56:03 MST 2022 aarch64 GNU/Linux

This does require guest kernel support (CONFIG_VIRTIO_FS=y), otherwise it will not work inside the guest; the script warns when there is a possibility this configuration is not set so the user is not surprised by the following error:

  / # mount -t virtiofs shared /mnt/shared
  mount: mounting shared on /mnt/shared failed: No such device

Closes: https://github.com/ClangBuiltLinux/boot-utils/issues/81 Link: https://gitlab.com/virtio-fs/virtiofsd