libkvm: fix warning issues post-r291406
- Fix -Wunused warnings with *_native detection handlers by marking kd __unused, except with arm/mips, where a slightly more complicated scheme is required to handle the native case vs the non-native case.
- Fix -Wmissing-variable-declarations warnings by marking struct kvm_arch objects static.
MFC after: 1 week
Sponsored by: Dell EMC Isilon