memory: allow concurrent backing reads

src/kernel/memory.h

@@ -192,6 +192,7 @@ bool FreeGuestMemory(uint64_t vaddr, uint64_t size);
 
 #if defined(KYTY_VIRTUAL_MEMORY_ALLOCATION_TESTS)
 void     TestBeforeNextBackingMap(callback_func_t callback);
+void     TestSetBackingReadCallback(callback_func_t callback);
 void     TestFailNextPhysicalMemoryUnmap();
 void     TestFailPhysicalMemoryUnmapAfter(uint32_t successful_unmaps);
 void     TestFailGuestBackingStoreUnmapAfter(uint32_t successful_unmaps);