virsh_setmem: Reduce churn for libvirtd restart

Rather than executing a libvirtd_restart each time the test is run
in order to ensure libvirtd is running - perform a check if it is
not running and try to start if not. If the start fails, then cause
test error.

Turns out if executed too quickly - the libvirtd restarts could end
causing the systemd restart to fail due to the underlying code perhaps
believing there's an infinite loop and issuing the following message
to /var/log/messages:

systemd[1]: libvirtd.service start request repeated too quickly,
refusing to start.