Fix misleading EC2 instance-id metadata debug URL

[jaehanbyun]

Summary

Update the EC2 datasource debug message to show the actual metadata endpoint used for instance-id.

The datasource currently logs:

$MDURL/instance-id

but ec2metadata --instance-id resolves the request to:

$MDURL/meta-data/instance-id

This can mislead users debugging CirrOS boot logs because copying the logged URL returns 404, while the actual metadata endpoint works.

Testing

sh -n src/lib/cirros/ds/ec2
git diff --check

Fixes #134