[PATCH v2] virsh: Remove <backingStore> when changing cdrom media source

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Since the code is changing the source image path by modifying the
existing XML snippet the <backingStore> stays in place.

As <backingStore> is relevant to the <source> part of the image, the
update of that part makes the element invalid.

CD/floppy images usually don't have a backing chain and the element is
currently ignored though but it might start being used in the future so
let's start behaving correctly.

Drop the <backingStore> subtree once we want to update the XML.

Before this patch, you'd get:
$ virsh change-media --eject --print-xml 10 hdc
<disk type="file" device="cdrom">
      <driver name="qemu" type="qcow2"/>

      <backingStore type="file" index="1">
        <format type="qcow2"/>
        <source file="/var/lib/libvirt/images/vm.1436949097"/>
        <backingStore/>
      </backingStore>
      <target dev="hdc" bus="ide"/>
      ...
    </disk>

After:

 $ virsh change-media --eject --print-xml 10 hdc
<disk type="file" device="cdrom">
      <driver name="qemu" type="qcow2"/>

      <target dev="hdc" bus="ide"/>
      ...
    </disk>
---

Notes:
    v2:
    - adapted to new version of code
    - fixed use after free from previous version
    - added more text to commit message

 tools/virsh-domain.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/virsh-domain.c b/tools/virsh-domain.c
index bf65a60..83edb21 100644
--- a/tools/virsh-domain.c
+++ b/tools/virsh-domain.c
@@ -11052,6 +11052,7 @@ virshUpdateDiskXML(xmlNodePtr disk_node,
 {
     xmlNodePtr tmp = NULL;
     xmlNodePtr source = NULL;
+    xmlNodePtr backingStore = NULL;
     xmlNodePtr target_node = NULL;
     xmlNodePtr text_node = NULL;
     char *device_type = NULL;
@@ -11092,13 +11093,22 @@ virshUpdateDiskXML(xmlNodePtr disk_node,
         if (xmlStrEqual(tmp->name, BAD_CAST "target"))
             target_node = tmp;

+        if (xmlStrEqual(tmp->name, BAD_CAST "backingStore"))
+            backingStore = tmp;
+
         /*
          * We've found all we needed.
          */
-        if (source && target_node)
+        if (source && target_node && backingStore)
             break;
     }

+    /* drop the <backingStore> subtree since it would become invalid */
+    if (backingStore) {
+        xmlUnlinkNode(backingStore);
+        xmlFreeNode(backingStore);
+    }
+
     if (type == VIRSH_UPDATE_DISK_XML_EJECT) {
         if (!source) {
             vshError(NULL, _("The disk device '%s' doesn't have media"), target);
-- 
2.6.2

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