vmm: Remove the per-vm cookie argument from vmmops taking a vcpu.

This requires storing a reference to the per-vm cookie in the
CPU-specific vCPU structure. Take advantage of this new field to
remove no-longer-needed function arguments in the CPU-specific
backends. In particular, stop passing the per-vm cookie to functions
that either don't use it or only use it for KTR traces.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37152