* src/conf/domain_conf.c:
  - Add virDomainControllerFind to find controller device by type
    and index.
  - Add virDomainControllerRemove to remove the controller device
    from maintained controler list.

* src/conf/domain_conf.h:
  - Declare the two new helpers.

* src/libvirt_private.syms:
  - Expose private symbols for the two new helpers.

* src/qemu/qemu_driver.c:
  - Support attach/detach controller device persistently

* src/qemu/qemu_hotplug.c:
  - Use the two helpers to simplify the codes.