Remove erroneous setting of return value to errno.

One error exit in virStorageBackendCreateBlockFrom was setting the
return value to errno. The convention for volume build functions is to
return 0 on success or -1 on failure. Not only was it not necessary to
set the return value (it defaults to -1, and is set to 0 when
everything has been successfully completed), in the case that some
caller were checking for < 0 rather than != 0, they would incorrectly
believe that it completed successfully.

src/storage/storage_backend.c

@@ -232,7 +232,6 @@ virStorageBackendCreateBlockFrom(virConnectPtr conn ATTRIBUTE_UNUSED,
     }
 
     if (fstat(fd, &st) == -1) {
-        ret = errno;
         virReportSystemError(errno, _("stat of '%s' failed"),
                              vol->target.path);
         goto cleanup;