bhyve: [snapshot] Rename 'structs' key with 'kern_structs'

[gusev.vitaliy_gmail.com]

Summary

Using key 'structs' is ambiguous. This section contains data
related to the kernel. It should use a more informative naming.

Sponsored by: vStack

Related to multiple devices support.

Test Plan

Note,

.meta file contents is changed:
From "structs": [
to "kern_structs": [

[corvink]

One general note: Please try to explain why a change is required in your commit message. "Related to xy" is not enough. It should explain why it is related to xy and what's the advantage of the patch.