Fix parameters check for ec2_vol

Split the error check into two error checks and delay the
checks so that listing existing volumes works more nicely.

The error check should check that:
* One and only one of volume_size id or name is set

This fix adds the 'only one' part of that check and provides
more useful error messages.
Merge pull request #7410 from willthames/ec2_vol_fix_error_check

Fix parameters check for ec2_vol