qemu-ga: skip starting on bare-metal device

[AndyChiang888]

The same firmware image may be deployed on either bare metal device or virtualized platforms (e.g., Proxmox VE).

On bare metal device, qemu-ga may still be started even though no virtio-serial channel is available, resulting in repeated attempts to access /dev/virtio-ports/org.qemu.guest_agent.0.

This causes continuous service respawning by procd and unnecessary log spam.

This commit adds a pre-check for /dev/virtio-ports to avoid starting qemu-ga when virtio-serial support is not present.

related logs:

daemon.err: qemu-ga[6259]: 1779119060.919739: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[6259]: 1779119060.919757: critical: failed to create guest agent channel
daemon.err: qemu-ga[6259]: 1779119060.919762: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[6259]: 1779119060.919791: critical: error initializing guest agent
daemon.err: qemu-ga[7016]: 1779119065.934454: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[7016]: 1779119065.934475: critical: failed to create guest agent channel
daemon.err: qemu-ga[7016]: 1779119065.934481: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[7016]: 1779119065.934486: critical: error initializing guest agent
daemon.err: qemu-ga[7313]: 1779119070.973746: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[7313]: 1779119070.973761: critical: failed to create guest agent channel
daemon.err: qemu-ga[7313]: 1779119070.973766: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[7313]: 1779119070.973791: critical: error initializing guest agent
daemon.err: qemu-ga[8164]: 1779119076.013910: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[8164]: 1779119076.013928: critical: failed to create guest agent channel
daemon.err: qemu-ga[8164]: 1779119076.013934: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[8164]: 1779119076.013939: critical: error initializing guest agent
daemon.err: qemu-ga[9531]: 1779119081.043780: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[9531]: 1779119081.043796: critical: failed to create guest agent channel
daemon.err: qemu-ga[9531]: 1779119081.043801: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[9531]: 1779119081.043806: critical: error initializing guest agent
daemon.err: qemu-ga[10502]: 1779119086.103792: critical: error opening channel '/dev/virtio-ports/org.qemu.guest_agent.0': No such file or directory
daemon.err: qemu-ga[10502]: 1779119086.103810: critical: failed to create guest agent channel
daemon.err: qemu-ga[10502]: 1779119086.103815: critical: failed to initialize guest agent channel
daemon.err: qemu-ga[10502]: 1779119086.103820: critical: error initializing guest agent
daemon.info: procd: Instance qemu-ga::instance1 s in a crash loop 6 crashes, 0 seconds since last crash

📦 Package Details

Maintainer: @
_{(You can find this by checking the history of the package Makefile.)}

Description:

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🧪 Run Testing Details

• OpenWrt Version: SNAPSHOT
• OpenWrt Target/Subtarget: x86/64
• OpenWrt Device: N100

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✅ Formalities

• I have reviewed the CONTRIBUTING.md file for detailed contributing guidelines.

If your PR contains a patch:

• It can be applied using git am
• It has been refreshed to avoid offsets, fuzzes, etc., using

  make package/<your-package>/refresh V=s

• It is structured in a way that it is potentially upstreamable
  _{(e.g., subject line, commit description, etc.)
  We must try to upstream patches to reduce maintenance burden.}