driver: Fix the detection of VM IP using prlctl command

[legal90]

It completes the fix introduced in #434

The previous statement with || operator always returned empty string if it was returned by read_guest_ip_dhcp. Now we are checking the result properly.
Also, I added the loop to make sure that we wait for the real IP instead of returning '' expecting the SSH client to retry.

[bineesh-n]

Still it can't detect IP from macvms. See the result of ssh-config :

testing vagrant ssh-config
Host default
  HostName
  User vagrant
  Port 22
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentityFile /Users/bineeshnadukkalam/.vagrant.d/boxes/mac13/0/parallels/vagrant_private_key
  IdentitiesOnly yes
  LogLevel FATAL

@legal90 I'm no ruby/vagrant expert, I think as per the implementation, 5 times trying for ip & wait 2 seconds, is not enough for macvms. Instead of ' ' you can try returning nil which will trigger vagrant's retry mechanism

[bineesh-n]

Hi @legal90 I think this has some additional code, which are not required. I've commented it. I can do the changes if you want.

IP came correctly with these changes though.

[legal90]

Thank you for the review, @bineesh-n !
Released as v2.3.1