suport device driver initialization model

According to PnP specification, Appendix B, Option ROMs
that support DDIM (device driver initialization model) should
have their memory space writeable.

KVM deviates from us here, by removing the IO_MEM_ROM flag,
to allow for PCI option ROMs (they require DDIM). However,
there's absolutely no reason we can't do the same.
Signed-off-by: NGlauber Costa <glommer@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/pc.c

@@ -920,8 +920,7 @@ static void pc_init1(ram_addr_t ram_size, int vga_ram_size,
 
     option_rom_offset = qemu_ram_alloc(0x20000);
     oprom_area_size = 0;
-    cpu_register_physical_memory(0xc0000, 0x20000,
-                                 option_rom_offset | IO_MEM_ROM);
+    cpu_register_physical_memory(0xc0000, 0x20000, option_rom_offset);
 
     if (using_vga) {
         /* VGA BIOS load */