fix(macos): gate no_fsync field and initializers for macOS only

The no_fsync feature is specific to macOS HVF framework. This commit:
1. Gates the no_fsync field in FsDeviceConfig with #[cfg(target_os = "macos")]
2. Updates all FsDeviceConfig initializers to conditionally set no_fsync
3. Gates the set_no_fsync method for macOS only
4. Gates the call site in builder.rs for macOS only

This fixes Linux build failures where no_fsync field doesn't exist
in the Linux Config struct.

Author: RoyLin

src/libkrun/src/lib.rs

@@ -667,6 +667,7 @@ pub unsafe extern "C" fn krun_set_root(ctx_id: u32, c_root_path: *const c_char)
                 shared_dir,
                 // Default to a conservative 512 MB window.
                 shm_size: Some(1 << 29),
+                #[cfg(target_os = "macos")]
                 no_fsync: false,
             });
         }
@@ -700,6 +701,7 @@ pub unsafe extern "C" fn krun_add_virtiofs(
                 fs_id: tag.to_string(),
                 shared_dir: path.to_string(),
                 shm_size: None,
+                #[cfg(target_os = "macos")]
                 no_fsync: false,
             });
         }
@@ -734,6 +736,7 @@ pub unsafe extern "C" fn krun_add_virtiofs2(
                 fs_id: tag.to_string(),
                 shared_dir: path.to_string(),
                 shm_size: Some(shm_size.try_into().unwrap()),
+                #[cfg(target_os = "macos")]
                 no_fsync: false,
             });
         }
@@ -2543,6 +2546,8 @@ pub unsafe extern "C" fn krun_set_root_disk_remount(
                 shared_dir: empty_root.to_string_lossy().into(),
                 // Default to a conservative 512 MB window.
                 shm_size: Some(1 << 29),
+                #[cfg(target_os = "macos")]
+                no_fsync: false,
             });
 
             ctx_cfg.set_block_root(device, fstype, options);

src/vmm/src/vmm_config/fs.rs

@@ -3,5 +3,6 @@ pub struct FsDeviceConfig {
     pub fs_id: String,
     pub shared_dir: String,
     pub shm_size: Option<usize>,
+    #[cfg(target_os = "macos")]
     pub no_fsync: bool,
 }