libvirtd: Explicit dependency on systemd-machined

libvirtd: Explicit dependency on systemd-machined

The libvirtd daemon uses systemd-machined D-Bus API when manipulating
domains. The systemd-machined is D-Bus activated on demand.

However, during system shutdown systemd-machined is stopped concurrently
with libvirtd and virsh users also doing their final cleanup may
transitively fail due to unavailability of systemd-machined. Example
error message

> libvirtd[1390]: 2017-12-20 18:55:56.182+0000: 32700: error : virSystemdTerminateMachine:503 : Refusing activation, D-Bus is shutting down.

To circumvent this we need to explicitly specify both ordering and
requirement dependency (to avoid late D-Bus activation) on
systemd-machined. See [1] for the dependency debate.

[1] https://lists.freedesktop.org/archives/systemd-devel/2018-January/040095.html