performance: switch to cPickle

profiling of the creation code path shows
that the impact of pickle is not negligible.

cPickle is a safe drop-in replacement, so this
patch switch to it for the sake of performance.

considering a 'monday morning' scenario (mass parallel
start of many VMs) we see ~5% improvement in startup time

before:
mean: 27.984s stdev=1.074s
best: 10.507s stdev=1.604s
worst: 42.711s stdev=1.996s
total: 895.479s stdev=34.375s

after:
mean: 26.423s stdev=1.413s
best: 9.785s stdev=1.669s
worst: 40.833s stdev=2.452s
total: 845.523s stdev=45.218s

pickle is used to save the VM params for checkpointing,
so the benefits should affect, possibly with lower extent,
to all VM operation flows.

Change-Id: I7b4b7d600c8432c948b795adad0459c318c8164b
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25860
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>