I use this script to backup my QEMU/KVM/libVirt virtual machines. The script requires KVM 2.1+ since it uses the live blockcommit mode. This means the data in the snapshot disk is rolled back into the original instead of the other way around. Script does NOT handle spaces in paths.

vm-backup.sh

#!/bin/bash
#

BACKUPDEST="$1"
DOMAIN="$2"
MAXBACKUPS="$3"

if [ -z "$BACKUPDEST" -o -z "$DOMAIN" ]; then
    echo "Usage: ./vm-backup <backup-folder> <domain> [max-backups]"
    exit 1
fi

if [ -z "$MAXBACKUPS" ]; then
    MAXBACKUPS=6
fi

echo "Beginning backup for $DOMAIN"

#
# Generate the backup path
#
BACKUPDATE=`date "+%Y-%m-%d.%H%M%S"`
BACKUPDOMAIN="$BACKUPDEST/$DOMAIN"
BACKUP="$BACKUPDOMAIN/$BACKUPDATE"
mkdir -p "$BACKUP"

#
# Get the list of targets (disks) and the image paths.
#
TARGETS=`virsh domblklist "$DOMAIN" --details | grep ^file | awk '{print $3}'`
IMAGES=`virsh domblklist "$DOMAIN" --details | grep ^file | awk '{print $4}'`

#
# Create the snapshot.
#
DISKSPEC=""
for t in $TARGETS; do
    DISKSPEC="$DISKSPEC --diskspec $t,snapshot=external"
done
virsh snapshot-create-as --domain "$DOMAIN" --name backup --no-metadata \
	--atomic --disk-only $DISKSPEC >/dev/null
if [ $? -ne 0 ]; then
    echo "Failed to create snapshot for $DOMAIN"
    exit 1
fi

#
# Copy disk images
#
for t in $IMAGES; do
    NAME=`basename "$t"`
    cp "$t" "$BACKUP"/"$NAME"
done

#
# Merge changes back.
#
BACKUPIMAGES=`virsh domblklist "$DOMAIN" --details | grep ^file | awk '{print $4}'`
for t in $TARGETS; do
    virsh blockcommit "$DOMAIN" "$t" --active --pivot >/dev/null
    if [ $? -ne 0 ]; then
        echo "Could not merge changes for disk $t of $DOMAIN. VM may be in invalid state."
        exit 1
    fi
done

#
# Cleanup left over backup images.
#
for t in $BACKUPIMAGES; do
    rm -f "$t"
done

#
# Dump the configuration information.
#
virsh dumpxml "$DOMAIN" >"$BACKUP/$DOMAIN.xml"

#
# Cleanup older backups.
#
LIST=`ls -r1 "$BACKUPDOMAIN" | grep -E '^[0-9]{4}-[0-9]{2}-[0-9]{2}\.[0-9]+$'`
i=1
for b in $LIST; do
    if [ $i -gt "$MAXBACKUPS" ]; then
        echo "Removing old backup "`basename $b`
        rm -rf "$b"
    fi

    i=$[$i+1]
done

echo "Finished backup"
echo ""

#!/bin/bash

# Set the language to English so virsh does it's output
# in English as well
LANG=en_US

# Define the script name, this is used with systemd-cat to
# identify this script in the journald output
SCRIPTNAME=kvm-backup

# List domains
DOMAINS=$(virsh list | tail -n +3 | awk '{print $2}')

# Loop over the domains found above and do the
# actual backup

for DOMAIN in $DOMAINS; do

	echo "Starting backup for $DOMAIN on $(date +'%d-%m-%Y %H:%M:%S')" | systemd-cat -t $SCRIPTNAME

	# Generate the backup folder URI - this is something you should
	# change/check
	BACKUPFOLDER=/mnt/$DOMAIN/$(date +%d-%m-%Y)
	mkdir -p $BACKUPFOLDER

	# Get the target disk
	TARGETS=$(virsh domblklist $DOMAIN --details | grep disk | awk '{print $3}')

	# Get the image page
	IMAGES=$(virsh domblklist $DOMAIN --details | grep disk | awk '{print $4}')

	# Create the snapshot/disk specification
	DISKSPEC=""

	for TARGET in $TARGETS; do
		DISKSPEC="$DISKSPEC --diskspec $TARGET,snapshot=external"
	done

	virsh snapshot-create-as --domain $DOMAIN --name "backup-$DOMAIN" --no-metadata --atomic --disk-only $DISKSPEC 1>/dev/null 2>&1

	if [ $? -ne 0 ]; then
		echo "Failed to create snapshot for $DOMAIN" | systemd-cat -t $SCRIPTNAME
		exit 1
	fi

	# Copy disk image
	for IMAGE in $IMAGES; do
		NAME=$(basename $IMAGE)
		cp $IMAGE $BACKUPFOLDER/$NAME
	done

	# Merge changes back
	BACKUPIMAGES=$(virsh domblklist $DOMAIN --details | grep disk | awk '{print $4}')

	for TARGET in $TARGETS; do
		virsh blockcommit $DOMAIN $TARGET --active --pivot 1>/dev/null 2>&1

		if [ $? -ne 0 ]; then
			echo "Could not merge changes for disk of $TARGET of $DOMAIN. VM may be in invalid state." | systemd-cat -t $SCRIPTNAME
			exit 1
		fi
	done

	# Cleanup left over backups
	for BACKUP in $BACKUPIMAGES; do
		rm -f $BACKUP
	done

	# Dump the configuration information.
	virsh dumpxml $DOMAIN > $BACKUPFOLDER/$DOMAIN.xml 1>/dev/null 2>&1

	echo "Finished backup of $DOMAIN at $(date +'%d-%m-%Y %H:%M:%S')" | systemd-cat -t $SCRIPTNAME
done

exit 0