mantle/qemu: Rework API to allow setting Config after creation

[cgwalters]

First, we don't hard require a config, so there's no reason
to force people to pass the empty string if they don't want one.

Second, this is prep for further work on 9p mounts which want
to build up a config and qemu arguments simultaneously.

[jlebon]

Yup, makes a lot of sense to me! (This actually came up in a convo I had with @sohankunkerkar yesterday about how to test coreos/ignition#903; we'll want a way to boot a VM without any Ignition config.)

/lgtm

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[cgwalters]

(This actually came up in a convo I had with @sohankunkerkar yesterday about how to test coreos/ignition#903; we'll want a way to boot a VM without any Ignition config.)

To be clear that worked before this too, you just had to pass "" for the config path.