Remove the current libvirtd pidfile handling code, in favour of
calling out to the new APIs. This ensures libvirtd's pidfile
handling is crashsafe
This also means that the non-root libvirtd instances (for handling
qemu:///session URIs) can now safely use pidfiles without racing
* daemon/libvirtd.c: Switch to use virPidFileAcquire and
virPidFileRelease