Skip any debconf prompting if user and group are already there

to not interfere with existing intallations

debian/libvirt-daemon-system.NEWS

-libvirt (2.4.0-2uidgid3) UNRELEASED; urgency=medium
+libvirt (2.5.0-2~) unstable; urgency=medium
 
   libvirt-daemon-system now uses the allocated uid and gid 64055
   for the libvirt-qemu user and group on new installations, when

debian/libvirt-daemon-system.config

 #!/bin/sh -e
 
+# Only perform this check/warning if the libvirt user groups aren't there
+# already
+if getent passwd libvirt-qemu >/dev/null \
+&& getent group  libvirt-qemu >/dev/null; then
+        exit 0
+fi
+
 # Source debconf library.
 . /usr/share/debconf/confmodule