libxl: Don't pass virDomainObjPtr to libxlBuildDomainConfig
    
    To make it easier to unit test, change libxlBuildDomainConfig
    so that it takes 'virDomainDefPtr' and 'libxl_ctx *' objects
    as separate parameters.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>