type and id, as compared to virtualised hook points which are now uniquely
identified by type, id and a vid (which for vimage is the pointer to the vnet
that the hhook resides in).
All hhook_head structs for both virtualised and non-virtualised hook points
coexist in hhook_head_list, and a separate list is maintained for hhook points
within each vnet to simplify some vimage-related housekeeping.
Reviewed by: scottl
MFC after: 1 week