commit e33cd67a9b51e8fcb4e084f40f500057b30b2786
Author: Jim Fehlig <jfehlig@suse.com>
Date:   Wed Jun 15 23:09:36 2016 -0600

    xenconfig: fix conversion of <driver> to backendtype
    
    When converting domXML to xen xl.cfg, backendtype should
    not be emitted if <driver> is not specified. Moreover,
    <driver name='file'/> should be converted to backendtype
    qdisk, similar to handling of <driver> in libxlMakeDisk()
    in libxl_conf.c.
    
    Prior to this change, connectDomainXMLToNative would
    produce incorrect xl.cfg when the input domXML contained
    <driver name='file'/>
    
    domXML:
    
      <disk type="file" device="disk">
        <driver name="file"/>
        <source file="/image/file/path"/>
        <target dev="xvda" bus="xen"/>
      </disk>
    
    virsh domxml-to-native xen-xl domXML
    
    disk = [ "format=raw,vdev=xvda,access=rw,backendtype=target=/image/file/path" ]
    
    xl create xl.cfg
    config parsing error in disk specification: unknown value
    for backendtype: near `target=/image/file/path' in
    `format=raw,vdev=xvda,access=rw,backendtype=target=/image/file/path'

Index: libvirt-1.3.5/src/xenconfig/xen_xl.c
===================================================================
--- libvirt-1.3.5.orig/src/xenconfig/xen_xl.c
+++ libvirt-1.3.5/src/xenconfig/xen_xl.c
@@ -752,13 +752,15 @@ xenFormatXLDisk(virConfValuePtr list, vi
     }
 
     /* backendtype */
-    virBufferAddLit(&buf, "backendtype=");
-    if (STREQ_NULLABLE(driver, "qemu"))
-        virBufferAddLit(&buf, "qdisk,");
-    else if (STREQ_NULLABLE(driver, "tap"))
-        virBufferAddLit(&buf, "tap,");
-    else if (STREQ_NULLABLE(driver, "phy"))
-        virBufferAddLit(&buf, "phy,");
+    if (driver) {
+        virBufferAddLit(&buf, "backendtype=");
+        if (STREQ(driver, "qemu") || STREQ(driver, "file"))
+            virBufferAddLit(&buf, "qdisk,");
+        else if (STREQ(driver, "tap"))
+            virBufferAddLit(&buf, "tap,");
+        else if (STREQ(driver, "phy"))
+            virBufferAddLit(&buf, "phy,");
+    }
 
     /* devtype */
     if (disk->device == VIR_DOMAIN_DISK_DEVICE_CDROM)