daemon: hivex: Check that hivex_commit is passed an absolute path (RHBZ#981683).

You will now get an error like:

libguestfs: error: hivex_commit: do_hivex_commit: path must start with a / character
hivex: Add some simple tests of the hivex_* APIs.

These are by no means comprehensive, but cover the basics.

The 'minimal' hive comes from hivex.
tests: Add basic Augeas tests.