curl: fix compilation on OpenBSD

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>

block/curl.c

@@ -377,7 +377,7 @@ static void curl_multi_check_completion(BDRVCURLState *s)
                     }
 
                     qemu_mutex_unlock(&s->mutex);
-                    acb->common.cb(acb->common.opaque, -EPROTO);
+                    acb->common.cb(acb->common.opaque, -EIO);
                     qemu_mutex_lock(&s->mutex);
                     qemu_aio_unref(acb);
                     state->acb[i] = NULL;