memfd: add file support

See "man memfd_create" for more information of what memfd is.

This adds support for memfd open files, that are not not memory mapped.

* We add a new kind of file: MEMFD.
* We add two image types MEMFD_FILE, and MEMFD_INODE.
  MEMFD_FILE contains usual file information (e.g., position).
  MEMFD_INODE contains the memfd name, and a shmid identifier
  referring to the content.
* We reuse the shmem facilities for dumping memfd content as it
  would be easier to support incremental checkpoints in the future.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>

lib/py/images/images.py

@@ -522,6 +522,8 @@ handlers = {
     'AUTOFS': entry_handler(pb.autofs_entry),
     'FILES': entry_handler(pb.file_entry),
     'CPUINFO': entry_handler(pb.cpuinfo_entry),
+    'MEMFD_FILE': entry_handler(pb.memfd_file_entry),
+    'MEMFD_INODE': entry_handler(pb.memfd_inode_entry),
 }