xen/hypfs: move per-node function pointers into a dedicated struct

xen/hypfs: move per-node function pointers into a dedicated struct

Move the function pointers currently stored in each hypfs node into a
dedicated structure in order to save some space for each node. This
will save even more space with additional callbacks added in future.

Provide some standard function vectors.

Instead of testing the write pointer to be not NULL provide a dummy
function just returning -EACCESS. ASSERT() all vector entries being
populated when adding a node. This avoids any potential problem (e.g.
pv domain privilege escalations) in case of calling a non populated
vector entry.

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