Allow per-driver config file for lock manager plugins

Allow per-driver config file for lock manager plugins

Allow a 'configFile' parameter to be passed into the lock
drivers to provide configuration. Wire up the QEMU driver
to pass in file names '/etc/libvirt/qemu-$NAME.conf
eg qemu-sanlock.conf

* src/locking/lock_driver.h, src/locking/lock_driver_nop.c,
  src/locking/lock_driver_sanlock.c, src/locking/lock_manager.c,
  src/locking/lock_manager.h: Add configFile parameter
* src/qemu/qemu_conf.c: Pass in configuration file path to
  lock driver plugins