QEMUSizedBuffer: only free qsb that qemu_bufopen allocated

Only free qsb that qemu_bufopen allocated, and also allow
qemu_bufopen accept qsb as input for write operation. It
will make the API more logical:
1.If you create the QEMUSizedBuffer yourself, you need to
  free it by using qsb_free() but not depends on other API
  like qemu_fclose.
2.allow qemu_bufopen() accept QEMUSizedBuffer as input for
  write operation, otherwise, it will be a little strange
  for this API won't accept the second parameter.

This brings API change, since there are only 3
users of this API currently, this change only impact the
first one which will be fixed in patch 2 of this patchset,
so I think it is safe to do this change.

1     70  tests/test-vmstate.c <<open_mem_file_read>>
            return qemu_bufopen("r", qsb);
2    404  tests/test-vmstate.c <<test_save_noskip>>
            QEMUFile *fsave = qemu_bufopen("w", NULL);
3    424  tests/test-vmstate.c <<test_save_skip>>
            QEMUFile *fsave = qemu_bufopen("w", NULL);

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Juan Quintela <quintela@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

migration/qemu-file-buf.c

@@ -395,6 +395,7 @@ QEMUSizedBuffer *qsb_clone(const QEMUSizedBuffer *qsb)
 typedef struct QEMUBuffer {
     QEMUSizedBuffer *qsb;
     QEMUFile *file;
+    bool qsb_allocated;
 } QEMUBuffer;
 
 static int buf_get_buffer(void *opaque, uint8_t *buf, int64_t pos, int size)
@@ -424,7 +425,9 @@ static int buf_close(void *opaque)
 {
     QEMUBuffer *s = opaque;
 
-    qsb_free(s->qsb);
+    if (s->qsb_allocated) {
+        qsb_free(s->qsb);
+    }
 
     g_free(s);
 
@@ -463,12 +466,11 @@ QEMUFile *qemu_bufopen(const char *mode, QEMUSizedBuffer *input)
     }
 
     s = g_malloc0(sizeof(QEMUBuffer));
-    if (mode[0] == 'r') {
-        s->qsb = input;
-    }
+    s->qsb = input;
 
     if (s->qsb == NULL) {
         s->qsb = qsb_create(NULL, 0);
+        s->qsb_allocated = true;
     }
     if (!s->qsb) {
         g_free(s);