golang/xenlight: fix StringList toC conversion

The current implementation of StringList.toC does not correctly account
for how libxl_string_list is expected to be laid out in C, which is clear
when one looks at libxl_string_list_length in libxl.c. In particular,
StringList.toC does not account for the extra memory that should be
allocated for the "sentinel" entry. And, when using the "slice trick" to
create a slice that can address C memory, the unsafe.Pointer conversion
should be on a C.libxl_string_list, not *C.libxl_string_list.

Fix these problems by (1) allocating an extra slot in the slice used to
address the C memory, and explicity set the last entry to nil so the C
memory will be zeroed out, and (2) dereferencing csl in the
unsafe.Pointer conversion.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

tools/golang/xenlight/xenlight.go

@@ -491,13 +491,14 @@ func (sl *StringList) fromC(csl *C.libxl_string_list) error {
 
 func (sl StringList) toC(csl *C.libxl_string_list) error {
 	var char *C.char
-	size := len(sl)
+	size := len(sl) + 1
 	*csl = (C.libxl_string_list)(C.malloc(C.ulong(size) * C.ulong(unsafe.Sizeof(char))))
-	clist := (*[1 << 30]*C.char)(unsafe.Pointer(csl))[:size:size]
+	clist := (*[1 << 30]*C.char)(unsafe.Pointer(*csl))[:size:size]
 
 	for i, v := range sl {
 		clist[i] = C.CString(v)
 	}
+	clist[len(clist)-1] = nil
 
 	return nil
 }