builder/virtualbox: fix type casting

/cc @pearkes - This fixes that crash

builder/virtualbox/common/ssh.go

@@ -12,7 +12,7 @@ func CommHost(state multistep.StateBag) (string, error) {
 }
 
 func SSHPort(state multistep.StateBag) (int, error) {
-	sshHostPort := state.Get("sshHostPort").(uint)
+	sshHostPort := state.Get("sshHostPort").(int)
 	return int(sshHostPort), nil
 }