Ignore non-zero exit from systemd-detect-virt in prepare script

As of b4b89ed3 we need the prepare script to exit zero on success.

We only use the stdout from this command, not the return code, for
determining if the target is a xen VM.

ROOT/DEBIAN/control

@@ -3,6 +3,6 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch94
+Version: 0.9-ch95
 Depends: zfs-dkms, postgresql-13 | postgresql-9.5 | postgresql-9.4 , liblockfile-simple-perl, libdbi-perl, libjson-perl, libzfs-perl-chem, libnet-openssh-perl, libdbd-pg-perl, mbuffer, rsync, nfs-kernel-server, pv, libwww-curl-perl
 Description: a backup system using ZFS (repository 'backup-scheduler')

ROOT/etc/chem-zfs-backup-server/zfs-rsync.d/prepare

@@ -54,7 +54,11 @@ fi
 # provided by it.
 # systemd-detect-virt is present on all our Ubuntu VMs so is hopefully a 
 # reliable method to use
+# NB this returns non-zero on a non-virtual machine so we don't mind
+# the return value
+set +e
 VIRT_TYPE=$(ssh root@$SERVER systemd-detect-virt 2>/dev/null)
+set -e
 if [ "$VIRT_TYPE" == "xen" ] ; then
 cat<<EOF >> $CONFDIR/$SERVER/exclude
 /lib/modules