Trace every libvirt call in the KVM provider at V(1), visible in the integration test

A tracingAPI decorator wraps the 17-method libvirt seam and logs one
"libvirt call" line per RPC after it returns: method, the identifiers it
was given, curated results, duration, and on failure the error with its
libvirt code, name and message. At V(2) the same line carries the domain
and volume XML, metadata and addresses; upload bodies are never logged.
The pool applies it in call() when V(1) is enabled and logs connect,
dial-failure and drop events, taking the caller's logger explicitly like
every other helper in the package.

The integration test installs a testr logger at KVM_TEST_LOG_LEVEL
(default 1) so the trace interleaves with t.Log output, which also ends
the "log.SetLogger(...) was never called" warning. Docs gained a Logging
section and the test env var; the KVM provider execution log records run
4 (guest hang at GRUB) that prompted this.

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

internal/provider/kvm/errors_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"reflect"
 	"testing"
 
 	libvirt "github.com/digitalocean/go-libvirt"
@@ -67,3 +68,26 @@ func TestHasCode(t *testing.T) {
 		t.Error("io.EOF classified as a libvirt error")
 	}
 }
+
+func TestLibvirtErrorFields(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		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"}},
+		{"transport error", io.EOF, nil},
+		{"cancelled", context.Canceled, nil},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got := libvirtErrorFields(tc.err)
+			if !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("libvirtErrorFields() = %#v, want %#v", got, tc.want)
+			}
+		})
+	}
+}