vmm: fix restore of TSC offset

After suspend/resume Ubuntu 20.04 and 22.04 installer can hang if
tsc-early clocksource has a big skew.

Reviewed by: corvink, jhb
Fixes: a7db532e3a6f83067b342f569b56076d011f8a1e ("vmm: Simplify saving of absolute TSC values in snapshots.")
MFC after: 2 weeks
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D38474

(cherry picked from commit 281b496f22d1d666b2ef1e54985432627ec434a4)