migration: Report when bdrv_inactivate_all fails

If the bdrv_inactivate_all fails near the end of the migration,
the migration will fail and often the only diagnostics in the log
are an I/O error which you can't distinguish from an error on
the socket connection.

Add an error so we know when it's actually a block problem.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170822170212.27347-1-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/savevm.c

@@ -1157,6 +1157,8 @@ int qemu_savevm_state_complete_precopy(QEMUFile *f, bool iterable_only,
          * bdrv_invalidate_cache_all() on the other end won't fail. */
         ret = bdrv_inactivate_all();
         if (ret) {
+            error_report("%s: bdrv_inactivate_all() failed (%d)",
+                         __func__, ret);
             qemu_file_set_error(f, ret);
             return ret;
         }