Use min_ram of original image for snapshot, even with VHD

When snapshotting an instance there seems to be no reason that min_ram
can't be set based off of the original image in all cases.  This relaxes
the restriction that VHD backed instances set min_ram for snapshots
based on the current instance.

Bug 1133524

Change-Id: I3779c48d7325f3f0bde496f43a70017fe79d2835

nova/compute/api.py

@@ -1644,13 +1644,13 @@ def _get_minram_mindisk_params(self, context, instance):
 
         #disk format of vhd is non-shrinkable
         if orig_image.get('disk_format') == 'vhd':
-            min_ram = instance['instance_type']['memory_mb']
             min_disk = instance['instance_type']['root_gb']
         else:
             #set new image values to the original image values
-            min_ram = orig_image.get('min_ram')
             min_disk = orig_image.get('min_disk')
 
+        min_ram = orig_image.get('min_ram')
+
         return min_ram, min_disk
 
     def _get_block_device_info(self, context, instance_uuid):

nova/tests/compute/test_compute.py

@@ -4663,8 +4663,8 @@ def fake_show(meh, context, id):
     def test_snapshot_minram_mindisk_VHD(self):
         """Ensure a snapshots min_ram and min_disk are correct.
 
-        A snapshot of a non-shrinkable VHD should have min_ram
-        and min_disk set to that of the original instances flavor.
+        A snapshot of a non-shrinkable VHD should have min_disk
+        set to that of the original instances flavor.
         """
 
         self.fake_image.update(disk_format='vhd',
@@ -4678,7 +4678,7 @@ def test_snapshot_minram_mindisk_VHD(self):
 
         self.assertEqual(image['name'], 'snap1')
         instance_type = instance['instance_type']
-        self.assertEqual(image['min_ram'], instance_type['memory_mb'])
+        self.assertEqual(image['min_ram'], self.fake_image['min_ram'])
         self.assertEqual(image['min_disk'], instance_type['root_gb'])
         properties = image['properties']
         self.assertTrue('backup_type' not in properties)