Allow CD-ROM images to be mounted without restarting the simulator (or the Mac)

[mihaip]

We can do some minor surgery to cdrom.cpp to not require that files are present (https://github.com/mihaip/macemu/blob/089f77754b8afc53ac22f24e534f0702c3e09989/BasiliskII/src/cdrom.cpp#L298-L300) and then trigger CDROMMountVolume (https://github.com/mihaip/macemu/blob/089f77754b8afc53ac22f24e534f0702c3e09989/BasiliskII/src/cdrom.cpp#L322) when they are actually created.

Followup to 800b508a33fe0e7c56adc5e434f63573c99b0373