vcontainer: add host-side idle timeout with QMP shutdown

Implement reliable idle timeout for vmemres daemon mode using
host-side monitoring with QMP-based shutdown, and container-aware
idle detection via virtio-9p shared file.

Host-side changes (vrunner.sh):
  - Add -no-reboot flag to QEMU for clean exit semantics
  - Spawn background watchdog when daemon starts
  - Watchdog monitors activity file timestamp
  - Check interval scales to idle timeout (timeout/5, clamped 10-60s)
  - Read container status from shared file (guest writes via virtio-9p)
  - Only shutdown if no containers are running
  - Send QMP "quit" command for graceful shutdown
  - Watchdog auto-exits if QEMU dies (no zombie processes)
  - Touch activity file in daemon_send() for user activity tracking

Config changes (vcontainer-common.sh):
  - Add idle-timeout to build_runner_args() so it's always passed

Guest-side changes (vcontainer-init-common.sh):
  - Add watchdog that writes container status to /mnt/share/.containers_running
  - Host reads this file instead of socket commands (avoids output corruption)
  - Close inherited virtio-serial fd 3 in watchdog subshell to prevent leaks
  - Guest-side shutdown logic preserved but disabled (QMP more reliable)
  - Handle Yocto read-only-rootfs volatile directories (/var/volatile)

The shared file approach avoids sending container check commands through
the daemon socket, which previously caused output corruption on the
single-stream virtio-serial channel.

The idle timeout is configurable via: vdkr vconfig idle-timeout <secs>
Default: 1800 seconds (30 minutes)

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

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