vmm: save the current time of the RTC in a snapshot

[wanpengqian_gmail.com]

Summary

vrtc_snapshot() saved base_rtctime, the time the RTC had at
base_uptime. That pair is brought up to date when the guest reads or
writes the RTC and, once a second, while the alarm or the update
interrupt is enabled. A guest that does neither for hours has a base
that is hours old. A restore sets base_uptime to the uptime of the
restore, so the restored RTC was late by the age of the base.

Save the time the RTC shows when the snapshot is taken. The format of
the snapshot does not change.

Signed-off-by: Wanpeng Qian <[e-mail]>
Sponsored by: keelos.dev

Test Plan

main (f958aa7e7) with WITH_BHYVE_SNAPSHOT, FreeBSD 16.0-CURRENT guest (it reads the RTC when it boots and then leaves it alone; LAPIC event timer). The guest idles for 5 minutes, then bhyvectl --suspend, 30 seconds later bhyve -r, and bhyvectl --get-rtc-time 2 seconds after that:

	host time	RTC
before: guest up	05:39:11	05:39:10
before: at the suspend	05:44:11	05:44:10
before: after the restore	05:44:48	05:38:52
after: guest up	05:45:33	05:45:33
after: at the suspend	05:50:33	05:50:33
after: after the restore	05:51:10	05:50:36

Before the change the restored RTC is back at the time of the guest's last access plus the seconds since the restore; after it, it goes on from the time of the suspend.

The time during which the guest was suspended is not added, before or after: that is a matter of policy and left alone here.