agent: Do not define domains persistent in libvirt

We used to define domains persistent in libvirt, which caused XML definitions
to stay there after a reboot of the hypervisor.

We however don't do anything with those already defined domains, actually, we wipe
all defined domains when starting the agent.

Some users however reported that libvirt started these domains after a reboot
before the CloudStack agent was started.

By starting domains from the XML description and not defining them we prevent
them from ever being stored in libvirt.
Summary: KVM - undefine persistent VMs on stop

Detail: A previous patch fixed an issue where we are defining VMs to persist
locally on KVM hosts, which can cause issues if the agent isn't running and
libvirt decides to start the VM unbeknownst to cloudstack. The previous patch
stopped defining VMs as persistent. This patch adds compatibility for existing
cloudstack environments, removing the persistent definition on stop if needed.

BUG-ID: CLOUDSTACK-600
Signed-off-by: Marcus Sorensen <marcus@betterservers.com> 1363194656 -0600
Summary: KVM - remove harmless message about domain not found on VM stop

Detail: When we stop a VM, it's definition is no longer valid. Therefore, we
need to catch the exception thrown from libvirt in trying to lookup a
non-existent domain by UUID while trying to check if it's shut down.

BUG-ID:CLOUDSTACK-600
Signed-off-by: Marcus Sorensen <marcus@betterservers.com> 1363201066 -0600