qemu: blockcopy: Report error on image format detection failure

We tolerate image format detection during block copy in very specific
circumstances, but the code didn't error out on failure of the format
detection.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -18104,15 +18104,14 @@ qemuDomainBlockCopyCommon(virDomainObjPtr vm,
         if (!mirror_reuse) {
             mirror->format = disk->src->format;
         } else {
-            if (mirror_initialized &&
-                virStorageSourceIsLocalStorage(mirror)) {
-                /* If the user passed the REUSE_EXT flag, then either they
-                 * can also pass the RAW flag or use XML to tell us the format.
-                 * So if we get here, we assume it is safe for us to probe the
-                 * format from the file that we will be using.  */
-                mirror->format = virStorageFileProbeFormat(mirror->path, cfg->user,
-                                                           cfg->group);
-            } else {
+            /* If the user passed the REUSE_EXT flag, then either they
+             * can also pass the RAW flag or use XML to tell us the format.
+             * So if we get here, we assume it is safe for us to probe the
+             * format from the file that we will be using.  */
+            if (!mirror_initialized ||
+                !virStorageSourceIsLocalStorage(mirror) ||
+                (mirror->format = virStorageFileProbeFormat(mirror->path, cfg->user,
+                                                            cfg->group)) < 0) {
                 virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                                _("reused mirror destination format must be specified"));
                 goto endjob;