[libvirt] Couple qemu driver bugs after xml refactoring

[Cole Robinson]

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