libxl: Fix setting of disk backend

The libxl driver was setting the backend field of libxl_device_disk
structure to LIBXL_DISK_BACKEND_TAP when the driver element of disk
configuration was not specified.  This needlessly forces the use of
blktap driver, which may not be loaded in dom0

https://bugzilla.redhat.com/show_bug.cgi?id=912488

Ian Campbell suggested that LIBXL_DISK_BACKEND_UNKNOWN is a better
default in this case

https://www.redhat.com/archives/libvir-list/2013-February/msg01126.html
(cherry picked from commit 567779e5)

src/libxl/libxl_conf.c

@@ -547,9 +547,13 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
             return -1;
         }
     } else {
-        /* No driverName - default to raw/tap?? */
+        /*
+         * If driverName is not specified, default to raw as per
+         * xl-disk-configuration.txt in the xen documentation and let
+         * libxl pick a suitable backend.
+         */
         x_disk->format = LIBXL_DISK_FORMAT_RAW;
-        x_disk->backend = LIBXL_DISK_BACKEND_TAP;
+        x_disk->backend = LIBXL_DISK_BACKEND_UNKNOWN;
     }
 
     /* XXX is this right? */