To allow hypervisor drivers to assume that a lock driver impl
will be guaranteed to exist, provide a 'nop' impl that is
compiled into the library

* src/Makefile.am: Add nop driver
* src/locking/lock_driver_nop.c, src/locking/lock_driver_nop.h:
  Nop lock driver implementation
* src/locking/lock_manager.c: Enable direct access of 'nop'
  driver, instead of dlopen()ing it.

Define the basic framework lock manager plugins. The
basic plugin API for 3rd parties to implemented is
defined in

  src/locking/lock_driver.h

This allows dlopen()able modules for alternative locking
schemes, however, we do not install the header. This
requires lock plugins to be in-tree allowing changing of
the lock manager plugin API in future.

The libvirt code for loading & calling into plugins
is in

  src/locking/lock_manager.{c,h}

* include/libvirt/virterror.h, src/util/virterror.c: Add
  VIR_FROM_LOCKING
* src/locking/lock_driver.h: API for lock driver plugins
  to implement
* src/locking/lock_manager.c, src/locking/lock_manager.h:
  Internal API for managing locking
* src/Makefile.am: Add locking code