hw/9pfs: Add new virtfs option writeout=immediate skip host page cache

writeout=immediate implies the after pwritev we do a sync_file_range.

Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

fsdev/qemu-fsdev.h

@@ -41,6 +41,7 @@ typedef struct FsTypeEntry {
     char *fsdev_id;
     char *path;
     char *security_model;
+    int export_flags;
     FileOperations *ops;
 } FsTypeEntry;