[libvirt] Couple qemu driver bugs after xml refactoring

Hi all, I've hit a couple bugs in the qemu driver with the recent domain xml refactoring. I've debugged them but in both cases I'm not sure what the optimal solutions are, so I'm just laying them out here: 1) Previously defining a qemu guest without a 'listen' address specified in the graphics tag would default to 127.0.0.1 (hardcoded in qemu_driver->vncListen). Current xml doesn't set this default, and will build a qemu command line with an entry like '-vnc (null):1'. Not sure if the default should be set at the parsing level or the driver level. 2) If a cpuset isn't specified, def->cpumask is null. However qemudInitCpus from qemu_driver.c doesn't know how to handle this, causing libvirtd to crash. This function is also using QEMUD_CPUMASK_LEN which I think should be replaced with VIR_DOMAIN_CPUMASK_LEN. I tried the obvious fix of just skipping the dependent code if cpumask is null, but there still seemed to be problems and I didn't dig much deeper. Thanks, Cole