vmm: Fix compiling error with BHYVE_SNAPSHOT

The return values of copyin() and copyout() must be checked.
vm_snapshot_buf_cmp() is unused by the kernel and was incorrectly
implemented, so just remove it.

Reviewed by: markj
Sponsored by: vStack
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43754