The reason that no disk drives appear is that there is no controller that allows for connecting disk drives. Since 0.186, the peripheral expansion box (PEB) has to be explicitly plugged into the I/O port. When this is done, MAME offers its slots for connecting peripheral cards, one of them possibly hosting a disk controller.
As it seems, QMC does not update the slot list when a slot option has been set, which is essential at this point. When ioport is set to peb, it should automatically offer the seven slots "ioport:peb:slot2" to "ioport:peb:slot8". Once a slot has been set to a controller card, the disk drive options will reappear.