automate fs share between host+guest

[nickdesaulniers]

I know it's possible to create a 9p fs shared between guest and host. It would be awesome if we could automate the creation+reuse of such a fs when used with the -i flag.

@nathanchance also mentioned https://virtio-fs.gitlab.io/. If that works, then let's use that; the ends are more important to me than the means (so I don't care whether we ultimately use 9p or virtiofs). I'd like to be able to copy coverage and profile data out of a guest back to a host.

[nathanchance]

This is my implementation in my personal QEMU wrapper script:

nathanchance/env@71b7809

Basically, we start virtiofsd with the information about the shared folder then QEMU connects to it. virtiofsd should be readily available when QEMU is installed but it would be good to check, it is typically at /usr/libexec/virtiofsd.

If that looks acceptable, I can wire up similar support in here.