ec2_vol: correctly follow Amazon's volume name instructions.

Follow up to 8617b6df903e90c3c1a659df79de164625e0782f, reading [1]
suggests that EBS volumes should mounted at /dev/sd[f-p] while
instance store volumes are mounted at /dev/sd[b-e].

[1] http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html