savevm: teach qemu_fill_buffer to do partial refills

We will need on next patch to be able to lookahead on next patch

v2: rename "used" to "pending" (Alex Williams)

Signed-off-by: Juan Quintela<quintela@redhat.com>
Reviewed-by: NAnthony Liguori <aliguori@us.ibm.com>

savevm.c

@@ -455,6 +455,7 @@ void qemu_fflush(QEMUFile *f)
 static void qemu_fill_buffer(QEMUFile *f)
 {
     int len;
+    int pending;
 
     if (!f->get_buffer)
         return;
@@ -462,10 +463,17 @@ static void qemu_fill_buffer(QEMUFile *f)
     if (f->is_write)
         abort();
 
-    len = f->get_buffer(f->opaque, f->buf, f->buf_offset, IO_BUF_SIZE);
+    pending = f->buf_size - f->buf_index;
+    if (pending > 0) {
+        memmove(f->buf, f->buf + f->buf_index, pending);
+    }
+    f->buf_index = 0;
+    f->buf_size = pending;
+
+    len = f->get_buffer(f->opaque, f->buf + pending, f->buf_offset,
+                        IO_BUF_SIZE - pending);
     if (len > 0) {
-        f->buf_index = 0;
-        f->buf_size = len;
+        f->buf_size += len;
         f->buf_offset += len;
     } else if (len != -EAGAIN)
         f->has_error = 1;