Add option to use file backed guest memory

Port qemu-kvm's -mem-path and -mem-prealloc options. These are useful
for backing guest memory with huge pages via hugetlbfs.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
CC: john cooper <john.cooper@redhat.com>
diff --git a/cpu-all.h b/cpu-all.h
index 8488bfe..9823c24 100644
--- a/cpu-all.h
+++ b/cpu-all.h
@@ -847,6 +847,9 @@
 extern ram_addr_t ram_size;
 extern ram_addr_t last_ram_offset;
 
+extern const char *mem_path;
+extern int mem_prealloc;
+
 /* physical memory access */
 
 /* MMIO pages are identified by a combination of an IO device index and