Change microvm.host.enable to be false by default

[SuperSandro2000]

Just importing a module shouldn't change things. This likely requires other changes.

[astro]

As the host module is to be imported implicitly, this would break things for many users.

I am aware of the troubles with conditional module imports. I'm open for this if there's a compelling reason.

[SuperSandro2000]

I did this because I explicitly needed to disable the module when building an iso where it isn't useful and just takes up space, which was quite unexpected

https://gitea.c3d2.de/c3d2/nix-config/src/commit/e9a4c3de115b57fd8188d659c6c047be3cde9967/modules/microvm-host.nix#L26

[astro]

On the other hand, the host module won't get in the way but provides the microvm command.