xen/hypfs: add getsize() and findentry() callbacks to hypfs_funcs

xen/hypfs: add getsize() and findentry() callbacks to hypfs_funcs

Add a getsize() function pointer to struct hypfs_funcs for being able
to have dynamically filled entries without the need to take the hypfs
lock each time the contents are being generated.

For directories add a findentry callback to the vector and modify
hypfs_get_entry_rel() to use it. For its non-directory node counterpart
introduce the so far unused and hence missing ENOTDIR error code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>