implement simple memory snapshot mechanismus

Uses Copy on Write to make it posible to restore the memory state after a snapshot
was made. To restore all MemoryRegions created after the snapshot are removed.

qemu/unicorn_common.h

@@ -137,6 +137,8 @@ static inline void uc_common_init(struct uc_struct* uc)
     uc->memory_map_io = memory_map_io;
     uc->set_tlb = uc_set_tlb;
     uc->memory_mapping = find_memory_mapping;
+    uc->memory_filter_subregions = memory_region_filter_subregions;
+    uc->memory_cow = memory_cow;
 
     if (!uc->release)
         uc->release = release_common;