Because we're caching e.g. uuid and metrics objects for the Xend session, we can get an exception
when the VM changed the power state between since we cached the object. We now catch this exception
and reload uuid and metrics object in the handler.
Open Xend session only once, cache various intermediary Xen API
objects for faster access. This all is important for Qubes Manager, so that it
doesn'tintroduce too much load on Dom0's CPU when displaying the load chart.
