virt.test : Make file_transfer support vhostforce on and off

This patch makes test file_transfer support vhostforce on
and off.
Signed-off-by: NYunping Zheng <yunzheng@redhat.com>

tests/cfg/file_transfer.cfg

@@ -6,3 +6,9 @@
     variants:
         - remote:
             transfer_type = remote
+    variants:
+        - no_vhostforce:
+        - vhostforce_off:
+            netdev_extra_params_nic1 += ',vhostforce=off'
+        - vhostforce_on:
+            netdev_extra_params_nic1 += ',vhostforce=on'