add ssh agent support for qemu builder

builder/qemu/ssh.go

@@ -1,10 +1,15 @@
 package qemu
 
 import (
+	"fmt"
+	"net"
+	"os"
+
 	commonssh "github.com/hashicorp/packer/common/ssh"
-	"github.com/hashicorp/packer/communicator/ssh"
+	packerssh "github.com/hashicorp/packer/communicator/ssh"
 	"github.com/hashicorp/packer/helper/multistep"
 	gossh "golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
 )
 
 func commHost(state multistep.StateBag) (string, error) {
@@ -19,10 +24,29 @@ func commPort(state multistep.StateBag) (int, error) {
 func sshConfig(state multistep.StateBag) (*gossh.ClientConfig, error) {
 	config := state.Get("config").(*Config)
 
-	auth := []gossh.AuthMethod{
-		gossh.Password(config.Comm.SSHPassword),
-		gossh.KeyboardInteractive(
-			ssh.PasswordKeyboardInteractive(config.Comm.SSHPassword)),
+	var auth []gossh.AuthMethod
+
+	if config.Comm.SSHAgentAuth {
+		authSock := os.Getenv("SSH_AUTH_SOCK")
+		if authSock == "" {
+			return nil, fmt.Errorf("SSH_AUTH_SOCK is not set")
+		}
+
+		sshAgent, err := net.Dial("unix", authSock)
+		if err != nil {
+			return nil, fmt.Errorf("Cannot connect to SSH Agent socket %q: %s", authSock, err)
+		}
+		auth = []gossh.AuthMethod{
+			gossh.PublicKeysCallback(agent.NewClient(sshAgent).Signers),
+		}
+	}
+
+	if config.Comm.SSHPassword != "" {
+		auth = append(auth,
+			gossh.Password(config.Comm.SSHPassword),
+			gossh.KeyboardInteractive(
+				packerssh.PasswordKeyboardInteractive(config.Comm.SSHPassword)),
+		)
 	}
 
 	if config.Comm.SSHPrivateKey != "" {