bhyve: Fix some warnings in the snapshot code

• Qualify unexported symbols with "static".
• Drop some unnecessary and incorrect casts.
• Avoid arithmetic on void pointers.
• Avoid signed/unsigned comparisons in loops which use nitems() as a bound.

No functional change intended.

MFC after: 1 week

(cherry picked from commit c9faf6987406781debb06ec5e40aa1de6d11ac4f)