Vamiga: Snapshots cannot be loaded if no Rom is present

[dirkwhoffmann]

If the default machine configuration does not specify a Kickstart Rom, the emulator refuses to load a snapshot file.

[dirkwhoffmann]

Confirmed: Method 'isReady()' was called too early:

Old code (MyController.swift):

```
// Check if the Amiga is ready to power on
if amiga.isReady() {

        // Power on the Amiga
        amiga.powerOn()

        // Process attachment (if any)
        mydocument.mountAmigaAttachment()

        // Launch the emulator thread
        amiga.run()

    }

New code: 

  ```
        // Process attachment (if any)
        mydocument.mountAmigaAttachment()

        // Check if the Amiga is ready to power on
        if amiga.isReady() {

            // Power on the Amiga
            amiga.powerOn()

            // Launch the emulator thread
            amiga.run()

        }