vmm: Expose struct vcpu as an opaque type.

Pass a pointer to the current struct vcpu to the vcpu_init callback
and save this pointer in the CPU-specific vcpu structures.

Add routines to fetch a struct vcpu by index from a VM and to query
the VM and vcpuid from a struct vcpu.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37156