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>