Per default, kvm instances are suspended when the host OS shuts down. This is down through an init scripot (libvirt-guests) that comes out of the box with Fedora and RHEL 6.

To change the guests’ shutdown behaviour, have a look at /etc/sysconfig/libvirt-guests. If you would like to send an ACPI system_powerdown event to the guest (i.e. a regular shutdown) instead of suspending it, change the ON_SHUTDOWN and SHUTDOWN_TIMEOUT varibale to

[...]
ON_SHUTDOWN=shutdown
SHUTDOWN_TIMEOUT=60

Of course, ymmv with the timeout value.