agent: Depend on JNA 3.5.1 for the Agent

The upcoming version of libvirt-java will require at least JNA 3.4 for running properly.

This version of JNA is not packaged for any recent Debian or Ubuntu (or other distribution),
so we have to include this in our own dependencies.
kvm: Fix some maven warnings

Maven was giving some warning when building the KVM plugin.

The dependency plugin had duplicate configuration and a version was missing
for the shade plugin.