How can I dynamically create a VM with ansible?

Ansible supports a lot of different virtualization providers, both local and remote. Their documentation contains good examples of usage for each.

There is also nothing stopping you from calling virsh/virt-install via command, and using ansible to create your XML config, disk images, etc.

Something a little like you might do for managing virtual hosts on a webserver.

This does assume you can runs commands on the hypervisor, though.