vga_pci: Add API to map the Video BIOS

Here are two new functions to map and unmap the Video BIOS:

void * vga_pci_map_bios(device_t dev, size_t *size);
void   vga_pci_unmap_bios(device_t dev, void *bios);

The BIOS is either taken from the shadow copy made by the System BIOS at
boot time if the given device was used for the default display (i386,
amd64 and ia64 only), or from the PCI expansion ROM.

Additionally, one can determine if a given device was the default
display at boot time using the following new function:

void   vga_pci_unmap_bios(device_t dev, void *bios);