pam: partly fix write-only mode

In write-only mode, writes are forwarded to RAM, while reads should not be
handled (ie should return 0xff).
Assume that in this mode, no read access is ever done, as they shouldn't
give any sensible result.

So, in write-only mode, alias PAM region to RAM, instead of PCI memory
(which can even be mapped to some device!)

This fixes Award BIOS, which use this mode to shadow system BIOS and video BIOS.
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/pci-host/pam.c

@@ -68,7 +68,7 @@ void init_pam(DeviceState *dev, MemoryRegion *ram_memory,
     /* XXX: should distinguish read/write cases */
     memory_region_init_alias(&mem->alias[0], OBJECT(dev), "pam-pci", pci_address_space,
                              start, size);
-    memory_region_init_alias(&mem->alias[2], OBJECT(dev), "pam-pci", pci_address_space,
+    memory_region_init_alias(&mem->alias[2], OBJECT(dev), "pam-pci", ram_memory,
                              start, size);
 
     for (i = 0; i < 4; ++i) {