commit a1c9a81a318f3cd558faed7fc4c4933f95c07a07
Author: Jim Fehlig <jfehlig@suse.com>
Date:   Mon May 23 15:56:01 2016 -0600

    libxl: default to qemu driver for network disks
    
    Xen only supports network-based disks with the qemu (aka qdisk) driver.
    Set the driverName to 'qemu' in libxlDomainDeviceDefPostParse() if
    not already set. When starting a domain with network-based disks,
    ensure the driverName is 'qemu'.
    
    Resolves: https://bugzilla.opensuse.org/show_bug.cgi?id=981094

Index: libvirt-1.3.4/src/libxl/libxl_conf.c
===================================================================
--- libvirt-1.3.4.orig/src/libxl/libxl_conf.c
+++ libvirt-1.3.4/src/libxl/libxl_conf.c
@@ -1055,13 +1055,18 @@ libxlMakeNetworkDiskSrc(virStorageSource
 int
 libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
 {
-    const char *driver;
-    int format;
+    const char *driver = virDomainDiskGetDriver(l_disk);
+    int format = virDomainDiskGetFormat(l_disk);
     int actual_type = virStorageSourceGetActualType(l_disk->src);
 
     libxl_device_disk_init(x_disk);
 
     if (actual_type == VIR_STORAGE_TYPE_NETWORK) {
+        if (STRNEQ_NULLABLE(driver, "qemu")) {
+            virReportError(VIR_ERR_OPERATION_INVALID, "%s",
+                           _("only the 'qemu' driver can be used with network disks"));
+            return -1;
+        }
         if (libxlMakeNetworkDiskSrc(l_disk->src, &x_disk->pdev_path) < 0)
             return -1;
     } else {
@@ -1072,8 +1077,6 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk
     if (VIR_STRDUP(x_disk->vdev, l_disk->dst) < 0)
         return -1;
 
-    driver = virDomainDiskGetDriver(l_disk);
-    format = virDomainDiskGetFormat(l_disk);
     if (driver) {
         if (STREQ(driver, "tap") || STREQ(driver, "tap2")) {
             switch (format) {
Index: libvirt-1.3.4/src/libxl/libxl_domain.c
===================================================================
--- libvirt-1.3.4.orig/src/libxl/libxl_domain.c
+++ libvirt-1.3.4/src/libxl/libxl_domain.c
@@ -364,6 +364,18 @@ libxlDomainDeviceDefPostParse(virDomainD
         }
     }
 
+    /* for network-based disks, set 'qemu' as the default driver */
+    if (dev->type == VIR_DOMAIN_DEVICE_DISK) {
+        virDomainDiskDefPtr disk = dev->data.disk;
+        int actual_type = virStorageSourceGetActualType(disk->src);
+
+        if (actual_type == VIR_STORAGE_TYPE_NETWORK) {
+            if (!virDomainDiskGetDriver(disk) &&
+                virDomainDiskSetDriver(disk, "qemu") < 0)
+                return -1;
+        }
+    }
+
     return 0;
 }