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/errors.go

@@ -68,16 +68,13 @@ func logAPIError(log logr.Logger, op string, err error) {
 }
 
 // libvirtErrorFields renders a daemon-reported error as log key/values:
-// the numeric code, its constant name (ErrNoDomain, ...) and the daemon's
-// message. Nil for transport and other non-libvirt errors.
+// its constant name (ErrNoDomain, ...) and numeric code. The daemon's
+// message is already the error text. Nil for transport and other
+// non-libvirt errors.
 func libvirtErrorFields(err error) []any {
 	var lerr libvirt.Error
 	if !errors.As(err, &lerr) {
 		return nil
 	}
-	return []any{
-		"libvirtCode", lerr.Code,
-		"libvirtCodeName", libvirt.ErrorNumber(lerr.Code).String(),
-		"libvirtMessage", lerr.Message,
-	}
+	return []any{"libvirtError", libvirt.ErrorNumber(lerr.Code).String(), "libvirtCode", lerr.Code}
 }