Simplify neonvm memory API to remove memory slots

[sharnoff]

Problem description / Motivation

Once #1060 is complete, we'll still be defining memory amounts in "number of slots" with "memory slots" of a certain size (even though the underlying size in virtio-mem is fixed size).

This complicates the API.

Feature idea(s) / DoD

DoD: The only remaining API for defining memory limits / usage for VirtualMachine objects is one that doesn't reference "memory slots".

Implementation ideas

We can add a new nullable field: .spec.guest.memory, with fields min/use/max of type resource.Quantity.

We'd also make .spec.guest.{memorySlots,memorySlotSize} nullable, with the requirement that either memorySlots/memorySlotSize are set, or the new memory field is set, and not both.

And then we can gradually migrate from one to the other by recreating all VMs. (or alternately allowing in-place modifications, but that seems like more hassle than it's worth, TBH)