xl: fix xl config parser

Bug: xl lexical analyzer cannot parse keyword which has .(dot) character like vif.default.script

ref: 733b9c524d
Error log: /etc/xen/xl.conf:28: config parsing error near `.default.script="vif-bridge"': lexical error

Signed-off-by: Choonho Son <choonho.son@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Roger Pau Monne <roger.pau@citrix.com>

tools/libxl/libxlu_cfg_l.l

@@ -52,7 +52,7 @@ void xlu__cfg_yyset_column(int  column_no, yyscan_t yyscanner);
 
 %%
 
-[a-z][_0-9a-z]*         {
+[a-z][._0-9a-z]*         {
                           yylval->string= xlu__cfgl_strdup(ctx,yytext);
                           GOT(IDENT);
                         }