vtc: Use a space as the host:port separator in b58

If ${localhost} resolves to an IPv6 address the colon separator becomes
ambiguous.

bin/varnishtest/tests/b00058.vtc

@@ -5,8 +5,8 @@ feature cmd {getent hosts localhost && getent services http}
 varnish v1 -arg "-p vcc_feature=-err_unref" -vcl {
 	backend b1 {.host = "127.0.0.1";}
 	backend b2 {.host = "[::1]:8080";}
-	backend b3 {.host = "${localhost}:8081";}
-	backend b4 {.host = "${localhost}:http";}
+	backend b3 {.host = "${localhost} 8081";}
+	backend b4 {.host = "${localhost} http";}
 	backend b5 {.host = "127.0.0.1";.port = "8081";}
 	backend b6 {.host = "127.0.0.1";.port = "http";}
 }