bhyve: use pci_next() to save/restore pci devices

Current snapshot implementation doesn't support multiple devices with
similar type. For example, two virtio-blk or two CD-ROM-s, etc.

So the following configuration cannot be restored.

bhyve \
-s 3,virtio-blk,disk.img \
-s 4,virtio-blk,disk2.img

In some cases it is restored silently, but doesn't work. In some cases
it fails during restore stage.

This commit fixes that issue.

Reviewed by: corvink, rew
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40109