saft: Fix the argument and the return value of get_internal_disk()

The get_internal_disk() doesn't work on ARM platform. The argument should
be a device with a partition and the return value should be a device without
a partition.

BUG=chrome-os-partner:13323
TEST=Run a FAFT test on ARM platform without error.

Change-Id: Ib02df7a277bf17755d96370689255f2bf4a96d2f
Reviewed-on: https://gerrit.chromium.org/gerrit/31850
Reviewed-by: Vic Yang <victoryang@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>

chromeos_interface.py

diff --git a/chromeos_interface.py b/chromeos_interface.py
index 7f23a1b..b074aa8 100755
--- a/chromeos_interface.py
+++ b/chromeos_interface.py
@@ -377,7 +377,7 @@
             else:
                 return '/dev/sda'
         else:
-            return device
+            return self.strip_part(device)
 
     def get_root_part(self):
         '''Return a string, the name of root device with partition number'''

kernel_handler.py

diff --git a/kernel_handler.py b/kernel_handler.py
index 2def349..5f0d387 100644
--- a/kernel_handler.py
+++ b/kernel_handler.py
@@ -57,7 +57,8 @@
           internal_disk - decide whether to use internal kernel disk.
         '''
         if internal_disk:
-            target_device = self.chros_if.get_internal_disk(self.root_dev)
+            target_device = self.chros_if.get_internal_disk(
+                    self.chros_if.get_root_part())
         else:
             target_device = self.root_dev