Explicitly format the isa-fdc controller for newer q35 machines

Explicitly format the isa-fdc controller for newer q35 machines

Since QEMU commit ea96bc6 [1]:
i386: drop FDC in pc-q35-2.4+ if neither it nor floppy drives are wanted
the floppy controller is no longer implicit.

Specify it explicitly on the command line if the machine type version
is 2.4 or later.

Note that libvirt's floppy drives do not result in QEMU implying the
controller, because libvirt uses if=none instead of if=floppy.

https://bugzilla.redhat.com/show_bug.cgi?id=1227880

[1] http://git.qemu.org/?p=qemu.git;a=commitdiff;h=ea96bc6