Introduce two new methods for triggering controlled shutdown/reboot

Introduce two new methods for triggering controlled shutdown/reboot

The virDomainShutdownFlags and virDomainReboot APIs allow the caller
to request the operation is implemented via either acpi button press
or a guest agent. For containers, a couple of other methods make
sense, a message to /dev/initctl, and direct kill(SIGTERM|HUP) of
the container init process.

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