Delete PID file on exit

Register an exit notifier to remove the PID file. By the time atexit()
is called, qemu_write_pidfile() guarantees QEMU owns the PID file,
thus we could safely remove it when exiting.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180907121319.8607-4-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>