qga: Fix undefined C behavior

The QAPI struct GuestFileWhence has a comment about how we are
exploiting equivalent values between two different integer types
shared in a union. But C says behavior is undefined on assignments to
overlapping storage when the two types are not the same width, and
indeed, 'int64_t value' and 'enum QGASeek name' are very likely to be
different in width.  Utilize a temporary variable to fix things.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Fixes: 0b4b493
Fixes: Coverity CID 1421990
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
(cherry picked from commit a23f38a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/commands.c

@@ -482,10 +482,15 @@ GuestExec *qmp_guest_exec(const char *path,
  * the guest's SEEK_ constants.  */
 int ga_parse_whence(GuestFileWhence *whence, Error **errp)
 {
-    /* Exploit the fact that we picked values to match QGA_SEEK_*. */
+    /*
+     * Exploit the fact that we picked values to match QGA_SEEK_*;
+     * however, we have to use a temporary variable since the union
+     * members may have different size.
+     */
     if (whence->type == QTYPE_QSTRING) {
+        int value = whence->u.name;
         whence->type = QTYPE_QNUM;
-        whence->u.value = whence->u.name;
+        whence->u.value = value;
     }
     switch (whence->u.value) {
     case QGA_SEEK_SET: