guestfish is used to mount virtual machine file systems.

[root@et-03-01-srv-001 ~]# guestfish --rw -a  zesty-server-cloudimg-amd64.img

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

><fs> run
><fs> list-filesystems
/dev/sda: ext4
><fs>