selftests/vsock: add namespace tests for CID collisions

Add tests to verify CID collision rules across different vsock namespace
modes.

1. Two VMs with the same CID cannot start in different global namespaces
   (ns_global_same_cid_fails)
2. Two VMs with the same CID can start in different local namespaces
   (ns_local_same_cid_ok)
3. VMs with the same CID can coexist when one is in a global namespace
   and another is in a local namespace (ns_global_local_same_cid_ok and
   ns_local_global_same_cid_ok)

The tests ns_global_local_same_cid_ok and ns_local_global_same_cid_ok
make sure that ordering does not matter.

The tests use a shared helper function namespaces_can_boot_same_cid()
that attempts to start two VMs with identical CIDs in the specified
namespaces and verifies whether VM initialization failed or succeeded.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260121-vsock-vmtest-v16-10-2859a7512097@meta.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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