libvirt-guests.service: Improve description

libvirt-guests.service does both suspend *and* resume guests,
depending on whether it's being started or stopped: the
description should reflect this, to avoid confusing messages
during startup.

Replace "active" with "running" (to match virsh list's output)
and don't capitalize libvirt.

tools/libvirt-guests.service.in

 [Unit]
-Description=Suspend Active Libvirt Guests
+Description=Suspend/Resume Running libvirt Guests
 After=network.target libvirtd.service time-sync.target
 Documentation=man:libvirtd(8)
 Documentation=http://libvirt.org