propolis-standalone could migrate time better

https://github.com/oxidecomputer/propolis/pull/359 added support for exporting guest time, importing it on the recipient side, tolerating skew between boot times across the source and destination systems, and wired it up into propolis-server. this stuff is all great, but in the mean time propolis-standalone got a fairly simple treatment of guest boot_hrtime.

I hadn't noticed this for a while, but this morning I kernel panicked my dev machine (me problem) and a VM I exported at ~60 days of uptime (this morning) now EINVALs on import because over in vmm_data_write_vmm_time we're rejecting src->vt_boot_hrtime > hrtime and will continue to for the next.. approximately sixty days.

I think just stuffing a VmTimeData into propolis-standalone's VmGlobalState would be basically fine on the export and import sides, but I haven't tested it and I'm chasing down something else at the moment...

Start in bin/propolis-standalone/src/snapshot.rs around the guest boot_hrtime handling, then trace vmm_data_write_vmm_time and the existing VmTimeData and VmGlobalState paths. Compare export and import behavior with propolis-server's time migration support; done means a VM with long uptime can be exported and imported without the EINVAL caused by boot-time skew.