vga: Respect isa_mem_base when registering chain4 alias

This does not yet unbreak PPC (which has its own problems) but
potentially other non-x86 systems where isa_mem_base is != 0.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/vga.c

diff --git a/hw/vga.c b/hw/vga.c
index 7be5141..f9a6014 100644
--- a/hw/vga.c
+++ b/hw/vga.c
@@ -181,6 +181,7 @@
             size = 0x8000;
             break;
         }
+        base += isa_mem_base;
         region = g_malloc(sizeof(*region));
         memory_region_init_alias(region, "vga.chain4", &s->vram, offset, size);
         memory_region_add_subregion_overlap(s->legacy_address_space, base,