libxl: prefer qdisk for <driver name='file'>

The libxl driver currently sets the disk backend to
LIBXL_DISK_BACKEND_TAP when <driver name='file'> is specified
in the <disk> config.  qdisk should be prefered with this
configuration, otherwise existing configuration such as the
following, which worked with the old Xen driver, will not work
with the libxl driver

  <disk type='file' device='cdrom'>
    <driver name='file'/>
    <source file='/path/to/some/iso'/>
    <target dev='hdc' bus='ide'/>
    <readonly/>
  </disk>

In addition, tap performs poorly compared to qdisk.

src/libxl/libxl_conf.c

@@ -796,7 +796,7 @@ libxlMakeDisk(virDomainDiskDefPtr l_disk, libxl_device_disk *x_disk)
                 return -1;
             }
             x_disk->format = LIBXL_DISK_FORMAT_RAW;
-            x_disk->backend = LIBXL_DISK_BACKEND_TAP;
+            x_disk->backend = LIBXL_DISK_BACKEND_QDISK;
         } else if (STREQ(driver, "phy")) {
             if (format != VIR_STORAGE_FILE_NONE &&
                 format != VIR_STORAGE_FILE_RAW) {