qemu: raise an eror when using aio=native without cache=none

Qemu will fallback to aio=threads when the cache mode doesn't use
O_DIRECT, even if aio=native was explictly set.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1086704Signed-off-by: NGiuseppe Scrivano <gscrivan@redhat.com>

src/qemu/qemu_command.c

@@ -3477,6 +3477,16 @@ qemuBuildDriveStr(virConnectPtr conn,
             mode = qemuDiskCacheV1TypeToString(disk->cachemode);
         }
 
+        if (disk->iomode == VIR_DOMAIN_DISK_IO_NATIVE &&
+            disk->cachemode != VIR_DOMAIN_DISK_CACHE_DIRECTSYNC &&
+            disk->cachemode != VIR_DOMAIN_DISK_CACHE_DISABLE) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("native I/O needs either no disk cache "
+                             "or directsync cache mode, QEMU will fallback "
+                             "to aio=threads"));
+            goto error;
+        }
+
         virBufferAsprintf(&opt, ",cache=%s", mode);
     } else if (disk->src->shared && !disk->src->readonly) {
         virBufferAddLit(&opt, ",cache=off");