Reference: bnc#872789
This is an indexing problem created by virt-manager.  It knows not
to add two IDE disks of the same name (eg, 'hda' twice) or two Xen
disks of the same name (eg, 'xvda' twice) but with the different bus
types (ide vs xen) it added xvda with hda.  These disks were then
passed to qemu where it error'ed out with the disks having the same
index (in this case both are 0).

Index: virt-manager-4.1.0/virtinst/devices/disk.py
===================================================================
--- virt-manager-4.1.0.orig/virtinst/devices/disk.py
+++ virt-manager-4.1.0/virtinst/devices/disk.py
@@ -903,6 +903,17 @@ class DeviceDisk(Device):
         :returns: generated target
         """
         prefix, maxnode = self.get_target_prefix()
+        postfix_targets = []
+        if self.conn.is_xen():
+            prefixes = [ "hd", "xvd", "vd", "sd", "fd" ]
+            for x in skip_targets:
+                if x is None:
+                    continue
+                for p in prefixes:
+                    found = x.split(p,1)
+                    if found and len(found) == 2:
+                        postfix_targets.append(found[1])
+                        break
         skip_targets = [t for t in skip_targets if t and t.startswith(prefix)]
         skip_targets.sort()
 
@@ -910,11 +921,18 @@ class DeviceDisk(Device):
             first_found = None
 
             for i in range(maxnode):
-                gen_t = prefix + self.num_to_target(i + 1)
+                postfix = self.num_to_target(i + 1)
+                gen_t = prefix + postfix
+                if self.conn.is_xen() and postfix in postfix_targets:
+                    if gen_t in skip_targets:
+                        skip_targets.remove(gen_t)
+                    continue
                 if gen_t in skip_targets:
                     skip_targets.remove(gen_t)
                     continue
                 if not skip_targets:
+                    if first_found:
+                        return first_found
                     return gen_t
                 elif not first_found:
                     first_found = gen_t