Refactors libvirt dependencies out of guest_config and launcher

- Joins host_config and guest_config into a single class
- Encapsulates most dependencies on libvirt into a LibvirtManager
  class to make it easier to substitute it with a direct call to qemu.

Bug: 79170615
Test: Run locally
Change-Id: Ia4a160aa2c949bd928b9d32475f8d85a493d5f38