Make the libvirt trace compact: libvirt API names, one error field, terse test output

The first live run showed the trace working but noisy: testr printed
ts/level/msg and the per-run constants on every line, the error text
appeared twice, host appeared twice on image-cache lines, and the Go
method name was less useful than the libvirt function it maps to.

The tracer now logs rpc=virDomainLookupByName (go-libvirt's names are the
public API names without the vir prefix), error plus libvirtError (code
name) and libvirtCode, and image.go stops re-adding host. The integration
test replaces testr with a compact logr sink: time, logger name, the rpc
as headline, then fields, with provider/host/name omitted since the first
line of the run states them. Docs and execution log updated.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/provider/kvm/conn_test.go

@@ -586,7 +586,7 @@ func TestHostPool_call_tracesAtV1(t *testing.T) {
 		t.Fatalf("call() error = %v", err)
 	}
 	joined := strings.Join(snapshot(), "\n")
-	for _, want := range []string{`"msg"="libvirt host connected"`, `"address"="nas1:22"`, `"method"="StoragePoolLookupByName"`, `"pool"="default"`, "libvirt "} {
+	for _, want := range []string{`"msg"="libvirt host connected"`, `"address"="nas1:22"`, `"rpc"="virStoragePoolLookupByName"`, `"pool"="default"`, "libvirt "} {
 		if !strings.Contains(joined, want) {
 			t.Errorf("output missing %s:\n%s", want, joined)
 		}