qemu: Fix crash when updating media with shared device

Mimic the fix done in 02b90972 to fix crash by
accessing an already freed structure. Also copy the explaining comment why the
pointer can't be accessed any more.
(cherry picked from commit 43b6f304)

src/qemu/qemu_driver.c

@@ -6073,12 +6073,15 @@ qemuDomainChangeDiskMediaLive(virDomainObjPtr vm,
             goto end;
 
         ret = qemuDomainChangeEjectableMedia(driver, vm, disk, orig_disk, force);
-        if (ret == 0) {
-            dev->data.disk = NULL;
+        /* 'disk' must not be accessed now - it has been freed.
+         * 'orig_disk' now points to the new disk, while 'dev_copy'
+         * now points to the old disk */
+
         /* Need to remove the shared disk entry for the original
          * disk src if the operation is either ejecting or updating.
          */
-            if (orig_disk->src && STRNEQ_NULLABLE(orig_disk->src, disk->src))
+        if (ret == 0) {
+            dev->data.disk = NULL;
             ignore_value(qemuRemoveSharedDisk(driver, dev_copy->data.disk,
                                               vm->def->name));
         }