provider/ec2: volumes should be attached with name "xvdf" instead of "xvdf1" by default

[Andrew Wilkins]

When attaching a volume to a machine in AWS, you can specify the device name. Except in hvm instances you have the option of naming devices like "xvdf" or like "xvdf1"; in hvm instances only the latter is supported.

Some things, like ceph, don't like it when the device name has a number on the end. We should allocate without numbers by default, with a pool option to include them (for a greater number of devices) if the user needs them.