qemu: driver: Fix qemuDomainHelperGetVcpus for sparse vcpu topologies
    
    ce43cca0e refactored the helper to prepare it for sparse topologies but
    forgot to fix the iterator used to fill the structures. This would
    result into a weirdly sparse populated array and possible out of bounds
    access and crash once sparse vcpu topologies were allowed.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1369988