Move 9p mount logic into QemuBuilder/qemuexec

[cgwalters]

Drain another thing out of cmd-run. This is prep for making
cmd-run be a tiny wrapper around kola qemuexec.

Also, using 9p mounts can be very useful for testing; i.e. this
helps unlock having qemu-specific tests that bind mount in data
without copying.

(One thing I'm thinking for example is that we support pulling
container images from the host)

[cgwalters]

Depends #1284

[jlebon]

All these draining PRs are great, though it'd be good to discuss a bit what the end goal is. In my mind, I see e.g. cmd-kola and cmd-run as becoming just compat shorthands for kola and kola qemuexec respectively (and eventually just dropping them). Or do you see them as continuing to wrap some lower-level switches?

I like the trend of making kola easier to use and just going for that directly. For example, it Just Works now to kola spawn in a cosa workdir.

So here for example, if we go that way, we could teach kola that --srv switch as a shorthand for --bind-rw=... and drop it entirely from cmd-run.

[cgwalters]

I think we do want to get rid of the wrappers eventually, but I'm trying to preserve the existing UX to some degree because...well I use cosa run a lot and I'd have to retrain my fingers.

[cgwalters]

So here for example, if we go that way, we could teach kola that --srv switch as a shorthand for --bind-rw=... and drop it entirely from cmd-run.

That one makes sense yeah.

[jlebon]

Yup, much nicer! LGTM once rebased on top of #1291.
/approve

[cgwalters]

Rebased 🏄‍♂️

[jlebon]

/lgtm

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