venus: suballocate more for layering

Previously we suballocate only for host visible memory type to reduce the kvm mem slot usage. That is no longer an issue given the limit has been raised. However, we should still suballocate to make layering clients performant at this moment. So we'd just suballocate regardless of mem type. This change also increases the allowed suballocation size request from 64K to 128K, which makes layering clients happier.

The 2nd commit purely adds more tracepoints to make debugging and performance analysis easier.