* 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.