Ensure root device is listed in block device mappings

moto/ec2/responses/amis.py

@@ -115,7 +115,7 @@ DESCRIBE_IMAGES_RESPONSE = """<DescribeImagesResponse xmlns="http://ec2.amazonaw
           {% endif %}
           <description>{{ image.description }}</description>
           <rootDeviceType>ebs</rootDeviceType>
-          <rootDeviceName>/dev/sda</rootDeviceName>
+          <rootDeviceName>/dev/sda1</rootDeviceName>
           <blockDeviceMapping>
             <item>
               <deviceName>/dev/sda1</deviceName>

tests/test_ec2/test_amis.py

@@ -57,6 +57,10 @@ def test_ami_create_and_delete():
         "Auto-created snapshot for AMI {0}".format(image.id))
     snapshot.volume_id.should.equal(volume.id)
 
+    # root device should be in AMI's block device mappings
+    root_mapping = image.block_device_mapping.get(image.root_device_name)
+    root_mapping.should_not.be.none
+
     # Deregister
     with assert_raises(EC2ResponseError) as ex:
         success = conn.deregister_image(image_id, dry_run=True)