The functions for manipulating pidfiles are in util/util.{c,h}.
We will shortly be adding some further pidfile related functions.
To avoid further growing util.c, this moves the pidfile related
functions into a dedicated virpidfile.{c,h}. The functions are
also all renamed to have 'virPidFile' as their name prefix

* util/util.h, util/util.c: Remove all pidfile code
* util/virpidfile.c, util/virpidfile.h: Add new APIs for pidfile
  handling.
* lxc/lxc_controller.c, lxc/lxc_driver.c, network/bridge_driver.c,
  qemu/qemu_process.c: Add virpidfile.h include and adapt for API
  renames