Support PCI based option rom loading

Currently, we preload option roms into the option rom space in memory.  This
prevents DDIM from functioning correctly which severely limits the number
of roms we can support.

This patch introduces a pci_add_option_rom() which registers the
PCI_ROM_ADDRESS bar which points to our option rom.  It also converts over
the cirrus vga adapter, the rtl8139, virtio, and the e1000 to use this
new mechanism.

The result is that PXE boot functions even with three unique types of cards.

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