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_test.go

@@ -76,8 +76,8 @@ func TestLibvirtErrorFields(t *testing.T) {
 		err  error
 		want []any
 	}{
-		{"libvirt error", lvErr(libvirt.ErrNoDomain), []any{"libvirtCode", uint32(42), "libvirtCodeName", "ErrNoDomain", "libvirtMessage", "ErrNoDomain"}},
-		{"wrapped libvirt error", fmt.Errorf("looking up: %w", lvErr(libvirt.ErrNoDomain)), []any{"libvirtCode", uint32(42), "libvirtCodeName", "ErrNoDomain", "libvirtMessage", "ErrNoDomain"}},
+		{"libvirt error", lvErr(libvirt.ErrNoDomain), []any{"libvirtError", "ErrNoDomain", "libvirtCode", uint32(42)}},
+		{"wrapped libvirt error", fmt.Errorf("looking up: %w", lvErr(libvirt.ErrNoDomain)), []any{"libvirtError", "ErrNoDomain", "libvirtCode", uint32(42)}},
 		{"transport error", io.EOF, nil},
 		{"cancelled", context.Canceled, nil},
 	}