isa: add isa_address_space()

A helper that returns the address space used by ISA devices.  Useful
for getting rid of isa_mem_base, multiple ISA buses, or ISA buses behind
bridges.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/isa-bus.c b/hw/isa-bus.c
index 2765543..1cb497f 100644
--- a/hw/isa-bus.c
+++ b/hw/isa-bus.c
@@ -20,6 +20,7 @@
 #include "monitor.h"
 #include "sysbus.h"
 #include "isa.h"
+#include "exec-memory.h"
 
 struct ISABus {
     BusState qbus;
@@ -202,4 +203,9 @@
     return strdup(path);
 }
 
+MemoryRegion *isa_address_space(ISADevice *dev)
+{
+    return get_system_memory();
+}
+
 device_init(isabus_register_devices)