virtio-devices: block: add helpful message on failed sector 0 write

We ran the Microsoft Windows installer in CHV (via network + VNC) and
the installation always failed when the installer wanted to write the
partition table. Since recently, for very good reason, sector 0 writes
are disabled if the image type is not set explicitly and only
implicitly auto-detected as raw [0].

To ease troubleshooting, I've added a descriptive log message. It is a
little spammy, but it is what is required to help users to troubleshoot.

[0] 6ecdf90e22

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com

virtio-devices/src/block.rs

@@ -186,6 +186,9 @@ impl BlockEpollHandler {
         }
 
         if request_type == RequestType::Out && disable_sector0_writes && request.sector == 0 {
+            warn!(
+                "Attempting to write to sector 0 on a raw disk without specifying image_type=raw"
+            );
             return Err(ExecuteError::ReadOnly);
         }