Add support for floppy disk

* Floppy is implemented as -global isa-fdc.drive?=$drive

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Super class for AHCI and IDE buses

Pylint doens't like the original version, this implementation is better.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>