The virtio hardware emulation code uses fprintf(stderr, ...) error messages. Improve things slightly by moving to error_report() so error messages will be printed to the monitor, if present.
We want to handle device error states properly instead of bailing out with exit(1) but this series does not attempt to fix that yet. Leave virtio-9p for now where there are many cases of fprintf(stderr, ...) and development is still very active.
