qemu: Make SetVcpu command hotplug only

qemu: Make SetVcpu command hotplug only

Similar to the Set*Mem commands, this implementation was bogus and
misleading. Make it clear this is a hotplug only operation, and that the
hotplug piece isn't even implemented.

Also drop the overkill maxvcpus validation: we don't perform this check
at XML define time so clearly no one is missing it, and there is
always the risk that our info will be out of date, possibly preventing
legitimate CPU values.

Signed-off-by: Cole Robinson <crobinso@redhat.com>