Add support for shutdown / reboot APIs in LXC driver

Add support for shutdown / reboot APIs in LXC driver

Add support for doing controlled shutdown / reboot in the LXC
driver. The default behaviour is to try talking to /dev/initctl
inside the container's virtual root (/proc/$INITPID/root). This
works with sysvinit or systemd. If that file does not exist
then send SIGTERM (for shutdown) or SIGHUP (for reboot). These
signals are not any kind of particular standard for shutdown
or reboot, just something apps can choose to handle. The new
virDomainSendProcessSignal allows for sending custom signals.

We might allow the choice of SIGTERM/HUP to be configured for
LXC containers via the XML in the future.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>