qemu: improve shutdown defaults for session daemon

qemu: improve shutdown defaults for session daemon

Currently the session daemon will try a managed save on all VMs,
leaving them running if that fails.

This limits the managed save just to persistent VMs, as there will
usually not be any way to restore transient VMs later.

It also enables graceful shutdown and then forced poweroff, should
save fail for some reason.

These new defaults can be overridden in the config file if needed.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>