virt.test: modify vhost config for file_transfer test

Currently for non-MSIX guest, qemu will disable vhost_net automatically,
even vhost is enabled. using vhostfoce=on to force enable vhost-net.
This patch modify the test configuration, only test "vhostforce=on"
on non-MSIX guest.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #1121 from humanux/vhostforce

virt.test: modify vhost config for file_transfer test