tests: libxl: Fix a segfault when libxl configuration setup fails
    
    This commit fixes a segmentation fault caused by missing conditional to
    check if libxl configuration was properly created by the test. If the
    configuration was not properly created, libxlDriverConfigNew() function
    will return NULL and cause a segfault at cfg->caps = NULL during the
    cleanup.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Reviewed-by: Erik Skultety <eskultet@redhat.com>