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'

src/xenconfig/xen_xl.c

@@ -857,13 +857,15 @@ xenFormatXLDisk(virConfValuePtr list, virDomainDiskDefPtr disk)
     }
 
     /* 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)