tweak help text for EC2 AMI prefix option ("instance" => "image")

The vmbuilder option to specify the AMI prefix, currently listed as --ec2-name, has help text of

  Name for the EC2 instance

This should be corrected to something like:

  Prefix for bundled EC2 image files

The ec2-bundle-image --prefix option help text is:

  The filename prefix for bundled AMI files

We need to be clear on the difference between "instance" and "image" on EC2.

See also Bug #285165 where I request changing the option from --ec2-name to --ec2-prefix for clarity and consistency.