rom loader: make vga+rom loading configurable.

rom loader: make vga+rom loading configurable.

The rom_add_vga() and rom_add_option() macros are transformed into
functions.  They look at the new rom_enable_driver_roms variable
and only do something if it is set to non-zero, making vga+option rom
loading runtime option.  pc_init() sets rom_enable_driver_roms to 1.

With this in place we can move the rom loading calls from pc.c to the
individual drivers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>