bhyve: Reject unsupported checkpoints before saving RAM

[hayzam_gmail.com]

Summary

pci_snapshot() rejects devices without a pe_snapshot callback, but
vm_checkpoint() reaches that check only after truncating checkpoint
files, pausing the VM, and saving RAM and kernel state.

Check for unsupported PCI devices at the start of vm_checkpoint(),
before creating files or pausing the VM. This preserves existing
checkpoint files when the request cannot succeed.

Sponsored by: The FreeBSD Foundation

Test Plan

A/B tested on amd64 with the same snapshot-enabled VMM and an Alpine
3.24.3 VM with 1 GiB of RAM configured with virtio-input.

Ran 12 cases per build: checkpoint/suspend, MSI-X on/off, and absent
or existing checkpoint files.

Before: requests wrote 1 GiB of RAM plus kernel state and metadata,
then failed after 1.01-1.64 seconds.

After: requests failed in 4.8-28.7 ms without creating files or
changing existing contents, sizes, modification times etc..