util: forbid freeing const pointers

Now that we've finally fixed all the violators, it's time to
enforce that any pointer to a const object is never freed (it
is aliasing some other memory, where the non-const original
should be freed instead).  Alas, the code still needs a normal
vs. Coverity version, but at least we are still guaranteeing
that the macro call evaluates its argument exactly once.

I verified that we still get the following compiler warnings,
which in turn halts the build thanks to -Werror on gcc (hmm,
gcc 4.8.3's placement of the ^ for ?: type mismatch is a bit
off, but that's not our problem):

    int oops1 = 0;
    VIR_FREE(oops1);
    const char *oops2 = NULL;
    VIR_FREE(oops2);
    struct blah { int dummy; } oops3;
    VIR_FREE(oops3);

util/virauthconfig.c:159:35: error: pointer/integer type mismatch in conditional expression [-Werror]
     VIR_FREE(oops1);
                                   ^
util/virauthconfig.c:161:5: error: passing argument 1 of 'virFree' discards 'const' qualifier from pointer target type [-Werror]
     VIR_FREE(oops2);
     ^
In file included from util/virauthconfig.c:28:0:
util/viralloc.h:79:6: note: expected 'void *' but argument is of type 'const void *'
 void virFree(void *ptrptr) ATTRIBUTE_NONNULL(1);
      ^
util/virauthconfig.c:163:35: error: type mismatch in conditional expression
     VIR_FREE(oops3);
                                   ^

* src/util/viralloc.h (VIR_FREE): No longer cast away const.
* src/xenapi/xenapi_utils.c (xenSessionFree): Work around bogus
header.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/viralloc.h

@@ -548,18 +548,17 @@ void virFree(void *ptrptr) ATTRIBUTE_NONNULL(1);
  * This macro is safe to use on arguments with side effects.
  */
 # if !STATIC_ANALYSIS
-/* The ternary ensures that ptr is a pointer and not an integer type,
- * while evaluating ptr only once.  This gives us extra compiler
- * safety when compiling under gcc.  For now, we intentionally cast
- * away const, since a number of callers safely pass const char *.
+/* The ternary ensures that ptr is a non-const pointer and not an
+ * integer type, all while evaluating ptr only once.  This gives us
+ * extra compiler safety when compiling under gcc.
  */
-#  define VIR_FREE(ptr) virFree((void *) (1 ? (const void *) &(ptr) : (ptr)))
+#  define VIR_FREE(ptr) virFree(1 ? (void *) &(ptr) : (ptr))
 # else
 /* The Coverity static analyzer considers the else path of the "?:" and
  * flags the VIR_FREE() of the address of the address of memory as a
  * RESOURCE_LEAK resulting in numerous false positives (eg, VIR_FREE(&ptr))
  */
-#  define VIR_FREE(ptr) virFree((void *) &(ptr))
+#  define VIR_FREE(ptr) virFree(&(ptr))
 # endif
 
 void virAllocTestInit(void);

src/xenapi/xenapi_utils.c

@@ -44,13 +44,15 @@ void
 xenSessionFree(xen_session *session)
 {
     size_t i;
+    char *tmp;
     if (session->error_description != NULL) {
         for (i = 0; i < session->error_description_count; i++)
             VIR_FREE(session->error_description[i]);
         VIR_FREE(session->error_description);
     }
     /* The session_id member is type of 'const char *'. Sigh. */
-    VIR_FREE(session->session_id);
+    tmp = (char *)session->session_id;
+    VIR_FREE(tmp);
     VIR_FREE(session);
 }