Live definition was used to look up the disk index while persistent one
was indexed leading to a crash in qemuDomainGetBlockIoTune. Use the
correct def and report a nice error.

Unfortunately it's accessible via read-only connection, though it can
only crash libvirtd in the cases where the guest is hot-plugging disks
without reflecting those changes to the persistent definition.  So
avoiding hotplug, or doing hotplug where persistent is always modified
alongside live definition, will avoid the out-of-bounds access.

Introduced in: eca96694a (v0.9.8)
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1140724Reported-by: NLuyao Huang <lhuang@redhat.com>
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>