xl: Command line: Support xl vcpu-set --help

This ended with a literal sentinel.  Use COMMON_LONG_OPTIONS (which
mentions --help) instead.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/libxl/xl_cmdimpl.c

@@ -5409,7 +5409,7 @@ int main_vcpuset(int argc, char **argv)
 {
     static struct option opts[] = {
         {"ignore-host", 0, 0, 'i'},
-        {0, 0, 0, 0}
+        COMMON_LONG_OPTS
     };
     int opt, check_host = 1;