pc: fw_cfg: move ioport base constant to pc.h

Move BIOS_CFG_IOPORT define from pc.c to pc.h, and rename
it to FW_CFG_IO_BASE.

Cc: Marc Marí <markmb@redhat.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marc Marí <markmb@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1455906029-25565-3-git-send-email-somlo@cmu.edu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>