cli: Add support for multiple backends for "moby run"

- Move HyperKit code into a separate file. It should be compilable
  on all supported OSes now.
- Add a (optional) subcommand to "moby run" to select a backend
  i.e., "moby run hyperkit [options] [prefix]"
- On macOS the default is "hyperkit" so that:
  "moby run [options] [prefix]"
  just works
- Add enough command line parsing to make it easy to add new
  backends to the run command
Update help messages.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

src/cmd/moby/run.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+func runUsage() {
+	fmt.Printf("USAGE: %s run [backend] [options] [prefix]\n\n", os.Args[0])
+
+	fmt.Printf("'backend' specifies the run backend.\n")
+	fmt.Printf("If not specified the platform specific default will be used\n")
+	fmt.Printf("Supported backends are (default platform in brackets):\n")
+	fmt.Printf("  hyperkit [macOS]\n")
+	fmt.Printf("\n")
+	fmt.Printf("'options' are the backend specific options.\n")
+	fmt.Printf("See 'moby run [backend] --help' for details.\n\n")
+	fmt.Printf("'prefix' specifies the path to the VM image.\n")
+	fmt.Printf("It defaults to './moby'.\n")
+}
+
+func run(args []string) {
+	if len(args) < 1 {
+		runUsage()
+		os.Exit(1)
+	}
+
+	switch args[0] {
+	case "help", "-h", "-help", "--help":
+		runUsage()
+		os.Exit(0)
+	case "hyperkit":
+		runHyperKit(args[1:])
+	default:
+		switch runtime.GOOS {
+		case "darwin":
+			runHyperKit(args)
+		default:
+			log.Errorf("There currently is no default 'run' backend for your platform.")
+		}
+	}
+}