Check for Winexe when customizing

We used to check for the presence of the winexe executable in the
Windows __init__ function. This is not optimal. The winexe check
should be done during customization. The user should be able to
dump, upload or register an image without having Winexe installed.

image_creator/os_type/windows/__init__.py

@@ -131,11 +131,6 @@ class Windows(OSBase):
             raise FatalError(
                 'For windows support libguestfs 1.16.11 or above is required')
 
-        # Check if winexe is installed
-        if not WinEXE.is_installed():
-            raise FatalError(
-                "For windows support `Winexe' needs to be installed")
-
         device = self.image.g.part_to_dev(self.root)
 
         self.last_part_num = self.image.g.part_list(device)[-1]['part_num']
@@ -305,6 +300,12 @@ class Windows(OSBase):
     def do_sysprep(self):
         """Prepare system for image creation."""
 
+        # Check if winexe is installed
+        if not WinEXE.is_installed():
+            raise FatalError(
+                "Winexe not found! In order to be able to customize a Windows "
+                "image you need to have Winexe installed.")
+
         if self.syspreped:
             raise FatalError(
                 "Microsoft's System Preparation Tool has ran on the Image. "