vbox: fix compilation error
    
    clang complains about possibly uninitialized variable:
    
    vbox/vbox_snapshot_conf.c:1355:9: error: variable 'ret' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (!(xPathContext = xmlXPathNewContext(xml))) {
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    So init 'ret' with NULL.