qemu: process: Fix failure semantics for perf events

qemu: process: Fix failure semantics for perf events

For strange reasons if a perf event type was not supported or failed to
be enabled at VM start libvirt would ignore the failure.

On the other hand on restart if the event could not be re-enabled
libvirt would fail to reconnect to the VM and kill it.

Both don't make really sense. Fix it by failing to start the VM if the
event is not supported and change the event to disabled if it can't be
reconnected (unlikely).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1329045