Avoid re-formatting the pidfile path everytime we need it. Create
it once when starting the guest, and preserve it until the guest
is shutdown.

* src/libvirt_private.syms, src/util/util.c,
  src/util/util.h: Add virFileReadPidPath
* src/qemu/qemu_domain.h: Add pidfile field
* src/qemu/qemu_process.c: Store pidfile path in qemuDomainObjPrivate