CVE-2014-3633: qemu: blkiotune: Use correct definition when looking up disk

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>

(cherry picked from commit 3e745e8f)

Conflicts:
	src/qemu/qemu_driver.c - context due to fewer functions

src/qemu/qemu_driver.c

@@ -12179,9 +12179,13 @@ qemuDomainGetBlockIoTune(virDomainPtr dom,
     }
 
     if (flags & VIR_DOMAIN_AFFECT_CONFIG) {
-        int idx = virDomainDiskIndexByName(vm->def, disk, true);
-        if (idx < 0)
+        int idx = virDomainDiskIndexByName(persistentDef, disk, true);
+        if (idx < 0) {
+            virReportError(VIR_ERR_INVALID_ARG,
+                           _("disk '%s' was not found in the domain config"),
+                           disk);
             goto endjob;
+        }
         reply = persistentDef->disks[idx]->blkdeviotune;
     }