vmm: add prefault option in memory and memory-zone

The argument `prefault` is provided in MemoryManager, but it can
only be used by SGX and restore.
With prefault (MAP_POPULATE) been set, subsequent page faults will
decrease during running, although it will make boot slower.

This commit adds `prefault` in MemoryConfig and MemoryZoneConfig.
To resolve conflict between memory and restore, argument
`prefault` has been changed from `bool` to `Option<bool>`, when
its value is None, config from memory will be used, otherwise
argument in Option will be used.

Signed-off-by: Yu Li <liyu.yukiteru@bytedance.com>

vmm/src/vm.rs

@@ -762,7 +762,7 @@ impl Vm {
         let memory_manager = MemoryManager::new(
             vm.clone(),
             &config.lock().unwrap().memory.clone(),
-            false,
+            None,
             phys_bits,
             #[cfg(feature = "tdx")]
             tdx_enabled,
@@ -886,7 +886,7 @@ impl Vm {
         let memory_manager = MemoryManager::new(
             vm.clone(),
             &config.lock().unwrap().memory.clone(),
-            false,
+            None,
             phys_bits,
             #[cfg(feature = "tdx")]
             false,