Improve resize2fs call for root.img resize

1. Do not start GUI session - to reduce startup time
2. Use user="root" instead of sudo - not all templates have sudo
installed (for example fedora-21-minimal)
3. Add missing wait=True - otherwise VM may be shutdown during the
operation.

QubesOS/qubes-issues#1268

core-modules/001QubesResizableVm.py

@@ -58,8 +58,9 @@ class QubesResizableVmWithResize2fs(QubesResizableVm):
 
     def resize_root_img(self, size):
         super(QubesResizableVmWithResize2fs, self).resize_root_img(size)
-        self.start()
-        self.run("sudo resize2fs /dev/mapper/dmroot")
+        self.start(start_guid=False)
+        self.run("resize2fs /dev/mapper/dmroot", user="root", wait=True,
+                 gui=False)
         self.shutdown()