Pull root device name for each VM instead of hardcoding it to /dev/sda1

When we're setting delvol_on_destroy for the root EBS volume, we're assuming the device name is always /dev/sda1. This works with Ubuntu images, but stock CentOS 6.4 images have been set up to use /dev/sda (no 1) for their root devices. To make this work, I've updated the logic to pull the root device name using the DescribeImages API and use that over the hardcoded name.