Use the existing virtio snapshot framework to save and restore balloon
configuration, cached guest statistics and their held-buffer handshake,
and diagnostic counters. Retain guest accounting across restore without
serializing locks, host pointers, or a persistent PFN ownership table.

Balloon reclamation finishes synchronously before request completion,
so there is no background worker to pause. Snapshotting stops vCPUs and
shares the IPC thread used for balloon management.

Remove the bhyve.8 note that checkpointing a VM with this device is not
supported.

Sponsored by: The FreeBSD Foundation

I tested this patch along with D60235 (diff 188422). Recompiled and relinked snapshot-enabled bhyve with -Werror, using a checkpoint-enabled VMM with INVARIANTS and WITNESS.

Passed an Alpine 3.24.3 UEFI/MSI-X suspend/restore cycle with two vCPUs, 1 GiB RAM, and a 256 MiB balloon. Target, actual size, and diagnostics survived restore unchanged. All 16 statistics fields refreshed, subsequent 512/128/0 MiB targets converged, and both guest memory-checking workers continued without corruption or discard errors. The same configuration stalled on fresh statistics without the BAR fix.