Use getent to find if a group exists, not grep

Change-Id: Ia181f41ae122dc5e5d3d33633dfd1d0e7420f8a9
diff --git a/lib/nova b/lib/nova
index 6445a07..29238d4 100644
--- a/lib/nova
+++ b/lib/nova
@@ -232,7 +232,7 @@
             LIBVIRT_DAEMON=libvirt-bin
         else
             # http://wiki.libvirt.org/page/SSHPolicyKitSetup
-            if ! grep ^libvirtd: /etc/group >/dev/null; then
+            if ! getent group libvirtd >/dev/null; then
                 sudo groupadd libvirtd
             fi
             sudo bash -c 'cat <<EOF >/etc/polkit-1/localauthority/50-local.d/50-libvirt-remote-access.pkla