kvm tools, seabios: Add "--firmware" option to "vm run"

This patch adds a "--firmware" command line option to "vm run". You can use
this to try to boot with SeaBIOS, for example:

  ./vm run --firmware=/usr/share/seabios/bios.bin \
	 --disk $HOME/images/debian_lenny_amd64_standard.qcow2

This doesn't boot yet for obvious reasons but at least people can now start to
play with external BIOS images easily.

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>