logging: reduce default boot logspam

[emilf]

Reduces default boot log volume (helpful for AI context + human scanning).

Changes

• Kernel defaults: log level Info (not Trace), verbose output off, hardware inventory printing off.
• QEMU headless default: drop extremely noisy -d int,cpu_reset; default to guest_errors with override via QEMU_DEBUG_FLAGS.
• Bootloader output: filter many success/progress lines when VERBOSE_OUTPUT=false.
• xHCI: demote some descriptor/port chatter to DEBUG.
• Docs: add docs/logging.md describing logging policy and knobs.

Before/after sanity: ./startQemu.sh headless 10 output dropped from ~1180 lines to ~159 lines in my captures.

Notes

• VERBOSE/DEBUG/TRACE can still be enabled via the monitor (loglevel ...) or by changing config constants.