When opt_xfer_len is zero, Linux ignores max_xfer_len erroneously.
While that obviously should be fixed, we do older guests a favor to
always filling in a value.
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170327142625.1249-1-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>