qemu-ga: skip starting on bare metal device

qemu-ga: skip starting on bare metal device

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.

Signed-off-by: Andy Chiang <redacted>