jsvm: create new isolate and enter to it using scope.

Otherwise you will get NULL for Isolate::GetCurrent() in new versions of
v8.

Now I've checked using next commands, and it passed on both versions of
v8: v3.14.5 (debian package) and from sources
9551c793c3706cbb023384b12fa769c4891aad03 ("Skip generated runtime tests
that require i18nsupport as needed") v3.26:
  $ echo 'HSET foo bar' | nc -q1000 localhost 9876
  $ echo $'*2\r\n$4\r\nHFOR\r\n$39\r\n(function(key, value) { throw "bya"; })\r\n' | nc -q1000 localhost 9876
  $ echo $'*2\r\n$4\r\nHFOR\r\n$76\r\n(function(key, value) { console.log("Key: " + key + ", value: " + value); })\r\n' | nc -q1000 localhost 9876