savevm: qemu_fille_buffer() used to return one error for reads of size 0.

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

savevm.c

diff --git a/savevm.c b/savevm.c
index f01838f..bee16c0 100644
--- a/savevm.c
+++ b/savevm.c
@@ -476,6 +476,8 @@
     if (len > 0) {
         f->buf_size += len;
         f->buf_offset += len;
+    } else if (len == 0) {
+        f->last_error = -EIO;
     } else if (len != -EAGAIN)
         f->last_error = len;
 }