qemudDomainSetVcpus: avoid NULL-deref on failed uuid look-up

qemudDomainSetVcpus: avoid NULL-deref on failed uuid look-up

* src/qemu/qemu_driver.c (qemudDomainSetVcpus): Upon look-up failure,
i.e., vm==NULL, goto cleanup, rather than to "endjob", superficially
since the latter would dereference vm, but more fundamentally because
we certainly don't want to call qemuDomainObjEndJob before we've
even attempted qemuDomainObjBeginJob.