kvm tools, seabios: Add support for SeaBIOS debugging output

SeaBIOS outputs debugging messages to special debug port ("0x0402"). This patch
unconditionally hooks the PIO port to LKVM so that when user specifies a
SeaBIOS image, you'll see this for "vm run":

  [penberg@tux kvm]$ ./vm run --firmware /usr/share/seabios/bios.bin
    # lkvm run -k ../../arch/x86/boot/bzImage -m 448 -c 4 --name guest-25983
  Start bios (version 0.6.0)
  Unable to unlock ram - bridge not found
  Ram Size=0x00100000 (0x0000000000000000 high)

  <hangs here>

Once we have SeaBIOS fully operational, we probably should hide the feature
under "--debug-seabios" command line option.

Acked-by Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Yang Bai <hamo.by@gmail.com>
Cc: Matt Evans <matt@ozlabs.org>
Cc: Ron Minnich <rminnich@gmail.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: John Floren <john@jfloren.net>
Cc: Sasha Levin <levinsasha928@gmail.com>
Cc: Asias He <asias.hejun@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>