EPROTO is not found in OpenBSD.   We usually use EIO when no better
errno is available, do that here too.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20170317152412.8472-1-pbonzini@redhat.com
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>