hostnamed: expose KernelVersion on the bus

This is needed to fix bug:

https://bugs.freedesktop.org/show_bug.cgi?id=76498

Reported-by: Zach <zachcook1991@gmail.com>
analyze: read host and system information from remote

This makes "systemd-analyze plot" read host information from remote.

While we are it show if this is a virtualized system.

https://bugs.freedesktop.org/show_bug.cgi?id=76498

Reported-by: Zach <zachcook1991@gmail.com>
update TODO