I use this script to backup my KVM - QEMU virtual machines. More detail: https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit

kvm_backup.sh

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

SCRIPTNAME=kvm-backup

# Local backup folder
LOCALBKFOLDER=/arc/vm

# Clean local backup folders older then days
ERASEBCKPDAYS=3

# Mounted network backup folder
NETBKFOLDER=/mnt/backup/VMs

echo "Start backup script at $(date +'%d-%m-%Y %H:%M:%S')" | systemd-cat -p info -t $SCRIPTNAME

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

for DOMAIN in $DOMAINS; do

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

	# Generate the backup folder URI - this is something you should
	# change/check
	BACKUPFOLDER=$LOCALBKFOLDER/$DOMAIN/$(date +%d-%m-%Y)
	NBACKUPFOLDER=$NETBKFOLDER/$DOMAIN
	echo "Create BACKUPFOLDER = $BACKUPFOLDER" | systemd-cat -p info -t $SCRIPTNAME
	mkdir -p $BACKUPFOLDER
        echo "Create NBACKUPFOLDER = $NBACKUPFOLDER" | systemd-cat -p info -t $SCRIPTNAME
	mkdir -p $NBACKUPFOLDER
	# Get the target disk
	TARGETS=$(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
	echo "Create snapshot for $DOMAIN, DISKSPEC = $DISKSPEC" | systemd-cat -p info -t $SCRIPTNAME
	virsh snapshot-create-as --domain $DOMAIN --name "backup-$DOMAIN" --no-metadata --atomic --disk-only $DISKSPEC

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

	# Copy disk image
	for IMAGE in $IMAGES; do
		NAME=$(basename $IMAGE)
		echo "Sync $IMAGE and $BACKUPFOLDER/$NAME" | systemd-cat -p info -t $SCRIPTNAME
		rsync -ahW --no-o --no-g --progress $IMAGE $BACKUPFOLDER/$NAME
	done

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

	for TARGET in $TARGETS; do
		 echo "Blockcommit for $DOMAIN, TARGET = $TARGET" | systemd-cat -p info -t $SCRIPTNAME
		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 -p warning -t $SCRIPTNAME
			exit 1
		fi
	done

	# Cleanup left over backups
	for BACKUP in $BACKUPIMAGES; do
        echo "Remove $BACKUP" | systemd-cat -p info -t $SCRIPTNAME
		rm -f $BACKUP
	done

	# Dump the configuration information.
	echo "Create xml for $DOMAIN  - $BACKUPFOLDER/$DOMAIN.xml" | systemd-cat -p info -t $SCRIPTNAME
	virsh dumpxml $DOMAIN > $BACKUPFOLDER/$DOMAIN.xml # 1>/dev/null 2>&1
        echo "Create xml for $DOMAIN  - $NBACKUPFOLDER/$DOMAIN.xml" | systemd-cat -p info -t $SCRIPTNAME
	virsh dumpxml $DOMAIN > $NBACKUPFOLDER/$DOMAIN.xml

	# Copy disk to network backup disk
        for IMAGE in $IMAGES; do
 	       NAME=$(basename $IMAGE)
	       echo "Copy $BACKUPFOLDER/$NAME to $NBACKUPFOLDER/$NAME" | systemd-cat -p info -t $SCRIPTNAME
	       cp $BACKUPFOLDER/$NAME $NBACKUPFOLDER/$NAME
	done

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

# Cleanup local backup folders
echo "Delete local backups $LOCALBKFOLDER older then $ERASEBCKPDAYS days" | systemd-cat -p info -t $SCRIPTNAME
find $LOCALBKFOLDER/* -ctime +$ERASEBCKPDAYS -type d -exec rm -r {} +

echo "Finished backup script at $(date +'%d-%m-%Y %H:%M:%S')" | systemd-cat -p info -t $SCRIPTNAME
exit 0

• Added fixes suggested in the comments https://gist.github.com/cabal95/e36c06e716d3328b512b
• Added extension logging. For view logs: journalctl -t kvm-backup -f
• Added cleanup local backup
• Added mounted network backup