Debian DomUs: Distinguish between $dist/updates and $dist-security

From Debian 11 Bullseye onwards, debian uses $dist-security instead of
$dist/updates as distribution part in /etc/apt/sources.list.

Mark those distributions with the old-style subdirectory path with
"security-subdir" in distributions.conf.

Thanks to Paul Wise for the bug report (and hence reminding me of
this) and for the suggestions on how to implement this (even if I
didn't follow them for the sake of simplicity and no additional
dependencies).

Closes: #972749

hooks/common.sh

@@ -422,3 +422,24 @@ installGentooPackage ()
     logMessage "NOTE: Not doing anything - this is a stub - FIXME"
 
 }
+
+
+
+
+#
+#  findDistributionConf path
+#
+# returns either a relative or an absolut path to distributions.conf
+#
+findDistributionsConf ()
+{
+    prefix=$1
+
+    if [ -f etc/distributions.conf ]; then
+        echo etc/distributions.conf
+    elif [ -f ${prefix:-}/etc/xen-tools/distributions.conf ]; then
+        echo ${prefix:-}/etc/xen-tools/distributions.conf
+    elif [ -f /etc/xen-tools/distributions.conf ]; then
+        echo /etc/xen-tools/distributions.conf
+    fi
+}