fixup register_memory_regions

Author: kalyazin

src/vmm/src/vstate/vm.rs

@@ -311,13 +311,13 @@ impl Vm {
                     );
                 }
 
-                addr as u64
+                Some(addr as u64)
             }
-            None => 0,
+            None => None,
         };
 
         for region in regions {
-            self.register_memory_region(region, Some(addr))?
+            self.register_memory_region(region, addr)?
         }
 
         Ok(())