Validate that instance exists and is EBS based before trying to launch it

*For Open Issue:* https://github.com/mitchellh/packer/issues/169

### Approach:
In the run source instance step make a call to ec2conn.Images using the source_ami id and no filters. Bails and throws errors on error response.  If an Image is returned confirm that its root_device_type is ebs, if not bails and throw errors.

### Source AMI IDs for testing:

**Ubuntu 10.04 LTS Instance-Store**

````
"source_ami": "ami-da0cf8b3"
````

**Ubuntu 10.04 LTS EBS**

````
"source_ami": "ami-4a0df923"
````

Link to base AMI the above IDs came from: https://aws.amazon.com/amis/ubuntu-10-04-lts-lucid-lynx-server-64-bit