vmm: Refactor storage of CPU-dependent per-vCPU data.

Rather than storing static arrays of per-vCPU data in the CPU-specific
per-VM structure, adopt a more dynamic model similar to that used to
manage CPU-specific per-VM data.

That is, add new vmmops methods to init and cleanup a single vCPU.
The init method returns a pointer that is stored in 'struct vcpu' as a
cookie pointer. This cookie pointer is now passed to other vmmops
callbacks in place of the integer index. The index is now only used
in KTR traces and when calling back into the CPU-independent layer.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37151