libvirt: out-of-bounds read access

Luyao Huang of Red Hat found that the qemu implementation of
virDomainGetBlockIoTune computed an index into the array of disks
for the live definition, then used it as the index into the array of
disks for the persistent definition, which could result into an
out-of-bounds read access in qemuDomainGetBlockIoTune().