Just to clarify, you're running Raspbian on QEMU, which is itself running on an Atom CPU, right?

The Pi is going to be loads faster in that case.

Yes, Raspbian has CDC ACM drivers built in. They're plug-and-play, so /dev/ttyACM0 should show up when you plug your adapter in if it really is CDC ACM. At least, it did for my 56k modem. I've never used a mobile broadband adapter, and I don't know if they usually demand special snowflake drivers or...

It isn't an emulator, but GDB will let you step through your program and look at memory locations, registers, the call stack, etc.