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/api/openapi/cloud-hypervisor.yaml

@@ -546,6 +546,9 @@ components:
         hotplugged_size:
           type: integer
           format: int64
+        prefault:
+          type: boolean
+          default: false
 
     MemoryConfig:
       required:
@@ -577,6 +580,9 @@ components:
         hugepage_size:
           type: integer
           format: int64
+        prefault:
+          type: boolean
+          default: false
         zones:
           type: array
           items: