vmm: Rework snapshotting of CPU-specific per-vCPU data.

Previously some per-vCPU state was saved in vmmops_snapshot and other
state was saved in vmmops_vcmx_snapshot. Consolidate all per-vCPU
state into the latter routine and rename the hook to the more generic
'vcpu_snapshot'. Note that the CPU-independent per-vCPU data is still
stored in a separate blob as well as the per-vCPU local APIC data.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37150