Update computeinstance-guide.md for current API and CLI

[tchughesiv]

Summary

docs/guides/developer/computeinstance-guide.md is out of date vs the current public API and CLI (validated during OSAC-244). Steps 1–3 (NetworkClass → VirtualNetwork → Subnet) are still correct; Step 4 and monitoring sections are not.

Evidence (current codebase)

• Public proto reserves removed fields: spec.subnet, spec.security_groups (fulfillment-service/proto/public/osac/public/v1/compute_instance_type.proto)
• Current field: spec.network_attachments (repeated NetworkAttachment)
• Public metadata uses singular creator / tenant, not creators / tenants (metadata_type.proto)
• CLI: osac create computeinstance --network-attachment "subnet=${SUBNET_ID}" (fulfillment-service/internal/cmd/cli/create/computeinstance/)

Requested updates

1. Replace stale YAML (spec.subnet, metadata.creators, metadata.tenants) with supported CLI:

   osac create computeinstance      --template osac.templates.ocp_virt_vm      --name <name>      --cores 2 --memory-gib 2 --boot-disk-size 10      --image quay.io/containerdisks/fedora:latest      --run-strategy Always      --network-attachment "subnet=${SUBNET_ID}"

   If YAML is kept as an alternate, use network_attachments and current metadata fields only.

2. Document subnet- namespace* (where KubeVirt VM/DataVolume objects land) vs tenant/installer namespace.

3. Document generated vm-* KubeVirt names vs fulfillment metadata.name.

4. Update monitoring: oc get vm,datavolume -n subnet-<id> (not only oc get vm -A).

5. Brief note for primary CUDN subnets: platform fabric egress may be required for boot disk import and runtime traffic; link docs/networking/setup-bpg-vrf-lite/README.md.