libkrun: vhost-user-vsock support

[dorindabassey]

libkrun: Add API constants and example for vhost-user vsock

Add public API constants for vhost-user vsock devices and example
usage in chroot_vm. The underlying support already exists via the
generic VhostUserDevice wrapper.

Example integration:

• Added --vhost-user-vsock option to chroot_vm
• Calls krun_disable_implicit_vsock() to avoid conflict with
  built-in device
• Skips TSI port mapping when vhost-user-vsock is active

[dorindabassey]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces support for vhost-user devices in libkrun, enabling the use of external backends for RNG, sound, and vsock devices. Key changes include the implementation of a generic VhostUserDevice wrapper, updates to the VM builder to support file-backed memory via memfd_create (necessary for vhost-user shared memory), and the addition of the krun_add_vhost_user_device C API. Feedback identifies a high-severity issue where the interrupt_monitor thread is not terminated during a device reset, potentially leading to resource leaks. Additionally, it is suggested to refactor repetitive error handling in the example code into a helper function to improve maintainability.

[gemini-code-assist]

The error handling logic for krun_add_vhost_user_device and krun_disable_implicit_vsock is duplicated across the setup for RNG, sound, and vsock devices. To improve maintainability and reduce code duplication, consider extracting this logic into a helper function. This aligns with the repository's priority on code readability.

Example helper:

static bool check_krun_error(int err, const char *msg)
{
    if (err) {
        errno = -err;
        perror(msg);
        return false;
    }
    return true;
}

References

1. Prioritize code readability and focused commits over micro-optimizations.