qemu: open serial log with O_APPEND via chardev

[sjmiller609]

Summary

Switch the QEMU serial console from -serial file:<path> to a chardev with append=on:

-chardev file,id=serial0,path=<path>,append=on -serial chardev:serial0

-serial file: opens the path with plain O_WRONLY|O_CREAT, so QEMU writes at its internal fd offset. If the file is externally truncated (e.g. by rotateLogIfNeeded's copytruncate), the next write lands at the stale offset, creating a sparse hole of NUL bytes from byte 0 to that offset. Downstream log readers chunk those NULs and choke.

append=on makes QEMU pass O_APPEND, so every write atomically seeks to EOF — post-truncate writes correctly resume at byte 0.

This only addresses QEMU. Cloud Hypervisor's serial.mode=File does not expose an append flag and needs a separate fix (likely switching to Socket/Pipe mode with hypeman owning the file fd).

Test plan

• go test ./lib/hypervisor/qemu/... passes
• Updated TestBuildArgs_SerialLog to assert the new chardev args
• Boot a VM with this build, run for long enough to trigger rotateLogIfNeeded, verify the post-rotation app.log has no sparse NUL hole

---

Note

Low Risk
Low risk: changes only QEMU CLI argument construction for serial logging and updates unit tests; main risk is QEMU arg compatibility/behavior differences across versions.

Overview
Switches QEMU serial logging from -serial file:<path> to a -chardev file,...,append=on plus -serial chardev:serial0, ensuring writes use O_APPEND and don’t create sparse NUL holes after external log truncation/rotation.

Updates TestBuildArgs_SerialLog to assert the new -chardev and chardev:serial0 arguments.

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[rgarcia]

lgtm