build: teach run script how to temporarily stop systemd units

build: teach run script how to temporarily stop systemd units

When testing locally built daemons on a systemd host there can be quite
a few systemd units that need temporarily stopping, and ideally
restarting after the test is complete. This becomes a massive burden
when modular daemons are running and you want to test libvirtd, as a
huge number of units need stopping.

The run script can facilitate this usage by looking at what units are
running and automatically stopping any that are known to conflict with
the daemon that is about to be run. This is only done when running as
root, since non-root libvirtd does not (currently) use systemd.

Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>