virtiofs: Fix writable virtiofs mounts

[cgwalters]

Regression from previous commit; we unconditionally added --readonly if virtiofsd supported it (which it doesn't yet in the Ubuntu 24.04 but does in Fedora 42+).

We want to support writable mounts.

It turns out the integration tests didn't catch this because they only looked at the permission bits.

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical regression that inadvertently made virtiofs mounts read-only in certain scenarios, hindering expected functionality. The core change involves introducing explicit control over the read-only behavior of virtiofs mounts through a new configuration flag. This ensures that mounts are writable by default unless specifically configured otherwise. Accompanying this fix are updated integration tests that now thoroughly validate both read-only and writable mount operations, providing robust coverage against similar issues.

Highlights

• Fix Writable Virtiofs Mounts: Resolved a regression that caused virtiofs mounts to be unconditionally read-only if the virtiofsd daemon supported the --readonly flag, even when writable mounts were intended.
• Explicit Read-Only Control: Introduced a readonly flag in VirtiofsConfig to explicitly control whether a virtiofs mount should be read-only, ensuring the --readonly argument is only passed to virtiofsd when requested.
• Enhanced Integration Tests: Refactored integration tests to accurately verify both read-only and writable virtiofs mount behavior, preventing future regressions related to mount permissions.

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[gemini-code-assist]

Code Review

This pull request fixes a regression that caused all virtiofs mounts to be read-only, even when they were intended to be writable. The core fix correctly adds the --readonly flag to virtiofsd only when a read-only mount is explicitly requested.

I appreciate the accompanying changes to the integration tests. Merging the two test helper functions into one and, more importantly, changing the tests to verify writability by actually attempting to write a file is a great improvement that will prevent similar regressions in the future.

I have one suggestion in the test code to improve robustness against special characters, but overall the changes are excellent and directly address the issue. Well done.