qemu: clarify message about block peek format restriction

Include the actual disk format in the error message to help the user see
what is wrong.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_driver.c

@@ -11544,8 +11544,9 @@ qemuDomainBlockPeek(virDomainPtr dom,
     }
 
     if (disk->src->format != VIR_STORAGE_FILE_RAW) {
-        virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
-                       _("peeking is supported only for RAW disks"));
+        virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
+                       _("peeking is only supported for disk with 'raw' format not '%s'"),
+                       virStorageFileFormatTypeToString(disk->src->format));
         goto cleanup;
     }