Xenon: support relative benchmark source paths.
Xenon: support relative benchmark source paths.

This means you can now write:

  `set source_dir ../some/relative/path`

And Xenon will resolve it to an absolute path, assuming that the base
directory is the directory from which Xenon was invoked.

Change-Id: I685ecdb7d16556e6508a9cecf4b6344a90207255

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin