libxl: pass driver config to libxlMakeDomBuildInfo

libxl: pass driver config to libxlMakeDomBuildInfo

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: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>