Add a space after comma.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-Id: <1427374663-10168-1-git-send-email-arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>