FIX #639 and #510 find powershell.exe

Signed-off-by: David Gageot <david@gageot.net>

drivers/hyperv/hyperv.go

@@ -130,6 +130,11 @@ func (d *Driver) GetState() (state.State, error) {
 
 // PreCreateCheck checks that the machine creation process can be started safely.
 func (d *Driver) PreCreateCheck() error {
+	// Check that powershell was found
+	if powershell == "" {
+		return ErrPowerShellNotFound
+	}
+
 	// Check that hyperv is installed
 	if err := hypervAvailable(); err != nil {
 		return err

drivers/hyperv/powershell.go

@@ -4,9 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"errors"
-	"os"
 	"os/exec"
-	"path/filepath"
 	"strings"
 
 	"fmt"
@@ -17,17 +15,13 @@ import (
 var powershell string
 
 var (
-	ErrNotAdministrator = errors.New("Hyper-v commands have to be run as an Administrator")
-	ErrNotInstalled     = errors.New("Hyper-V PowerShell Module is not available")
+	ErrPowerShellNotFound = errors.New("Powershell was not found in the path")
+	ErrNotAdministrator   = errors.New("Hyper-v commands have to be run as an Administrator")
+	ErrNotInstalled       = errors.New("Hyper-V PowerShell Module is not available")
 )
 
 func init() {
-	systemPath := strings.Split(os.Getenv("PATH"), ";")
-	for _, path := range systemPath {
-		if strings.Index(path, "WindowsPowerShell") != -1 {
-			powershell = filepath.Join(path, "powershell.exe")
-		}
-	}
+	powershell, _ = exec.LookPath("powershell.exe")
 }
 
 func cmdOut(args ...string) (string, error) {