The code how it is today, is totally painful to read and keep.
To begin with, the code is duplicated with the option rom loading
code that linux_boot and vga are already using.

This patch introduces a "bootable" state in NICInfo structure,
that we can use to keep track of whether or not a given nic should
be bootable, avoiding the introduction of yet another global state.

With that in hands, we move the code in vl.c to hw/pc.c, and use
the already existing infra structure to load those option roms.

Error checking code suggested by Mark McLoughlin

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