Validate the Idle-PC value and display an error if invalid

Validate the Idle-PC value entered in the settings and display an error if not valid.

https://community.gns3.com/message/6546#6546
https://community.gns3.com/thread/4607

This is performing a simple validation using the same regex which is used by the server code.
```
^(0x[0-9a-fA-F]+)?$
```