Linux Virtualize VMs may omit SPICE virtserialport, breaking clipboard functionality

**Describe the issue**
On Apple Silicon Macs with Linux guests in Virtualize mode, clipboard functionality may fail even when Clipboard sharing is enabled and:

* `spice-vdagentd` is running
* `/dev/virtio-ports/com.redhat.spice.0` exists in the guest

The following QEMU arguments resolve the issue:

````
-chardev spicevmc,id=vdagent,name=vdagent
-device virtserialport,chardev=vdagent,name=com.redhat.spice.0
````

This suggests that the Linux Virtualize template does not always create the SPICE virtserial channel required by the agent.

I have documented the underlying requirement upstream in the SPICE agent README here: https://github.com/utmapp/vd_agent/pull/1

**Configuration**
* UTM Version: 4.7.4 (115)
* macOS Version: 15.7.3
* Mac Chip: M1