vmm: Simplify saving of absolute TSC values in snapshots.

Read the current "now" TSC value and use it to compute absolute time
saved value in vm_snapshot_vcpus rather than iterating over vCPUs
multiple times in vm_snapshot_vm.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37146

(cherry picked from commit a7db532e3a6f83067b342f569b56076d011f8a1e)