Preparation for global nestedhvm configuration - libxlMakeDomBuildInfo
needs access to libxlDriverConfig.
No functional change.

Adjusting tests require slightly more mockup functions, because of
libxlDriverConfigNew() call.
Signed-off-by: NMarek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Reviewed-by: NJim Fehlig <jfehlig@suse.com>