The Requires relationship is very strong, in that it prevents
a unit from running unless all the units it Requires are
running as well.
This turns out to be a problem because we want to be able to
restart libvirtd at any time without having libvirt-guests
suspend or shutdown running guests.
Turn the Requires relationship into a Wants relationship:
this way starting libvirt-guests will cause systemd to (attempt
to) start libvirtd as well, but stopping or restarting libvirtd
will not alter libvirt-guests' running state.
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