Hide VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES where needed

Running unit test against a newer libvirt fails due to
VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES being always available.

This was mostly hidden as the old unit test globally deleted
the attribute during execution. Removing this side effect
and use mocking of attributes instead where needed.

Related-Bug: LP #1266534

Change-Id: Id0e6d8771f1a43f402b6323f0c0df38ee23e0fa8
Merge "Hide VIR_CONNECT_BASELINE_CPU_EXPAND_FEATURES where needed"