Script to gracefully power off KVM instances

When a node is running KVM instances and that node is rebooted or
shutdown, the gnt-noded daemon is finished and leaves the KVM instances
running. Latter on in the shutdown process, all remaining processes
receive SIGTERM as usual, meaning that the KVM instances are all
terminated, without running a proper shutdown procedure inside the
guests.

When using Xen, the xendomains script will take care of gracefully
powering down the instances, but for KVM there is nothing like that.

This patch adds an script that sends the "system_powerdown" command to
all running instances, and it could be useful for people using KVM.

This patch is a response for issue #126

Signed-off-by: Miguel Di Ciurcio Filho <miguel.filho@gmail.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>