Backup from inside a guest OS assumes backing up and recovering virtual machines similarly to physical machines. This functionality becomes available by using Acronis Backup & Recovery 11.5 Agent for Windows or Acronis Backup & Recovery 11.5 Agent for Linux.
For online backup and recovery, install Agent for Windows or Agent for Linux in the corresponding guest system. You can use bootable media to do off-line ("cold") backups and "bare metal" recovery to an empty virtual machine. Installing the software, backing up, and recovery are the same as with a physical machine.