Ansible: how to call module `add_host` for all hosts of the play

As you noted, there's a thing about add_host: BYPASS_HOST_LOOP = True.
So it's a kind of forced run_once.

If you don't mind running over hypervisors in sequential manner, you can simply use serial: 1:

- hosts: hypervisors
  serial: 1
  tasks:
    - shell: virsh list | awk 'NR>2' | awk '{print $2}'
      register: result_virsh
    - add_host:
        name: "{{ item }}"
        group: "guests"
      with_items: "{{ result_virsh.stdout_lines }}"

This ensures that every play batch consists of only one host, so add_host executes for every host.