storage_conf.c: avoid overflow upon use of "z" or "Z" (zebi) suffix

* src/storage_conf.c (virStorageSize): Don't try to compute 1024^7,
since it's too large for a 64-bit type.

src/storage_conf.c

@@ -919,12 +919,6 @@ virStorageSize(virConnectPtr conn,
                 1024ull;
             break;
 
-        case 'z':
-        case 'Z':
-            mult = 1024ull * 1024ull * 1024ull * 1024ull * 1024ull *
-                1024ull * 1024ull;
-            break;
-
         default:
             virStorageReportError(conn, VIR_ERR_XML_ERROR,
                                   _("unknown size units '%s'"), unit);