i386/pc_q35: don't insist on board FDC if there's no default floppy

i386/pc_q35: don't insist on board FDC if there's no default floppy

The "no_floppy = 1" machine class setting causes "default_floppy" in
main() to become zero. Consequently, default_drive() will not call
drive_add() and drive_new() for IF_FLOPPY, index=0, meaning that no
default floppy drive will be created for the virtual machine. In that
case, board code should also not insist on the creation of the
board-default FDC.

The board-default FDC will still be created if the user requests a floppy
drive with "-drive if=floppy".

Additionally, separate FDCs can be specified manually with "-device
isa-fdc". They allow the

  -device isa-fdc,driveA=...

syntax that is more flexible than the one required by the board-default
FDC:

  -global isa-fdc.driveA=...

This patch doesn't change the behavior observably, as all Q35 machine
types have "no_floppy = 0".

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: John Snow <jsnow@redhat.com>
Cc: "Gabriel L. Somlo" <gsomlo@gmail.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>