In daemons using pidfiles to protect against concurrent
execution there is a possibility that a crash may leave a stale
pidfile on disk, which then prevents later restart of the daemon.

To avoid this problem, introduce a pair of APIs which make
use of virFileLock to ensure crash-safe & race condition-safe
pidfile acquisition & releae

* src/libvirt_private.syms, src/util/virpidfile.c,
  src/util/virpidfile.h: Add virPidFileAcquire and virPidFileRelease