machine: share virtiofs systemd unit generation

[inknos]

Make GenerateSystemDFilesForVirtiofsMounts an agnostic function and move the code from apple to pkg/machine/volume_systemd.go.

MountVolumesToVM is now a no-op matching the AppleHV and LibKrun behaviour.

Continues from PR: #28736

Fixes: https://redhat.atlassian.net/browse/RUN-4742

Checklist

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• Referenced issues using Fixes: #00000 in commit message (if applicable)
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Does this PR introduce a user-facing change?

None

[inknos]

Note, with this change the existing machines will have to be stopped, removed and recreated, since they won't have the correct systemd files at init time. Is it considered a breaking change or just workaroundable? @baude @Honny1