virt-ls: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/virt-ls - from package: libguestfs-tools-c

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/virt-ls - from package: libguestfs-tools

You may install the required package with command apt-get install {package}

Error in other languages:
virt-ls: bevel nie gevind nie
virt-ls: comandă negăsită
virt-ls: comando não encontrado
virt-ls: commande introuvable
virt-ls: command not found
virt-ls: käsku ei ole
virt-ls: không tìm thấy lệnh
virt-ls: komanda nerasta
virt-ls: Kommando nicht gefunden.
virt-ls: kommandot finns inte
virt-ls: komut yok
virt-ls: nie znaleziono polecenia
virt-ls: níor aimsíodh an t-ordú
virt-ls: no se encontró la orden
virt-ls: no s'ha trobat l'ordre
virt-ls: opdracht niet gevonden
virt-ls: parancs nem található
virt-ls: perintah tidak ditemukan
virt-ls: príkaz nenájdený
virt-ls: příkaz nenalezen
virt-ls: команда не найдена
virt-ls: командата не е открита
virt-ls: 命令找不到
virt-ls: コマンドが見つかりません