qemu.tests: Add sr_iov_file_transfer test case.

This case use multi_vms_file_transfer.py script.
It test file transfer between vms with vf, pf.
Signed-off-by: NFeng Yang <fyang@redhat.com>

qemu/tests/cfg/sr_iov_file_transfer.cfg

+- sr_iov_file_transfer:
+    only vf_assignable 
+    type = multi_vms_file_transfer
+    filesize = 4000
+    transfer_timeout = 1000
+    vms += " vm2"
+    image_snapshot = yes
+    tmp_dir = /tmp/
+    repeat_time = 10
+    variants:
+        - vf:
+            pci_assignable = vf
+        - pf:
+            pci_assignable_nic1 = vf
+            device_name = eth1
+        - vf_pf_mixed:
+            pci_assignable_nic1_vm1 = pf
+            device_name_vm1 = eth1
+            pci_assignable_vm2 = vf