Fix parsing of SpotInstanceRequests responce when we also specify NetworkInterface

When you specify LaunchSpecification.NetworkInterface.* for [RequestSpotInstances](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-RequestSpotInstances.html)
response of [DescribeSpotInstanceRequests](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeSpotInstanceRequests.html) have additional elements 
```xml
<networkInterfaceSet>
   <item>
     <deviceIndex>0</deviceIndex>
     <subnetId>subnet-aa707382</subnetId>
     <description>test</description>
     <privateIpAddress>10.0.0.12</privateIpAddress>
     <groupSet>
       <item>
         <groupId>sg-f1612594</groupId>
       </item>
     </groupSet>
     <deleteOnTermination>true</deleteOnTermination>
   </item>
</networkInterfaceSet>
```
[Default xml response](http://pastebin.com/3FVeqmUB)
[With networking xml response](http://pastebin.com/c6BTeVtM)

With networking we does not parce instanceId well.
This pull request fix bug with parsing such cases.