REDHAT-BUG-2492681: Low severity KubeVirt vulnerability

A flaw was found in KubeVirt's downward metrics virtio-serial server in virt-handler. The server reads guest requests using textproto.Reader.ReadLine() in pkg/downwardmetrics/virtio-serial/server.go, which buffers input indefinitely until a newline character is received, with no length limit or read deadline. The server is started from dmetrics-manager.go and wired into the virt-handler process at cmd/virt-handler/virt-handler.go — it runs inside virt-handler's address space, not in the per-VM virt-launcher pod. A VM guest writing a continuous byte stream without newlines to the downward-metrics virtio-serial channel causes unbounded heap allocation in virt-handler until it is OOM-killed. Affected image: virt-handler-rhel9. Upstream fix should wrap the reader in io.LimitedReader with a small cap and add a per-read deadline.