fw_cfg: don't map the fw_cfg IO ports in fw_cfg_io_realize()

fw_cfg: don't map the fw_cfg IO ports in fw_cfg_io_realize()

As indicated by Laszlo it is a QOM bug for the realize() method to actually
map the device. Set up the IO regions within fw_cfg_io_realize() and defer
the mapping with sysbus_add_io() to the caller, as already done in
fw_cfg_init_mem_wide().

This makes the iobase and dma_iobase properties now obsolete so they can be
removed.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>