memory, xen: pass MemoryRegion to xen_ram_alloc()

Currently xen_ram_alloc() relies on ram_addr, which is going away.
Give it something else to use as a cookie.

Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/xen-all.c b/xen-all.c
index b5e28ab..b441fc1 100644
--- a/xen-all.c
+++ b/xen-all.c
@@ -181,7 +181,7 @@
     }
 }
 
-void xen_ram_alloc(ram_addr_t ram_addr, ram_addr_t size)
+void xen_ram_alloc(ram_addr_t ram_addr, ram_addr_t size, MemoryRegion *mr)
 {
     unsigned long nr_pfn;
     xen_pfn_t *pfn_list;