bhyve: rename 'structs' snapshot key with 'kern_structs'

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

Reviewed by: corvink, rew
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40104

(cherry picked from commit 50aef9f62c1be9e723169ef57920c3fca07bd2fb)