This patch adds a dummy legacy ISA device whose responsibility is to
deploy sgabios, an option rom for a serial graphics adapter.
The proposal is that this device is always-on when -nographics,
but can otherwise be enable in any setup when -device sga is used.

[v2: suggestions on qdev by Markus ]
[v3: cleanups and documentation, per list suggestions ]

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>