qemu.tests.cfg: Create config file for nfs_perf script

Signed-off-by: NYiqiao Pu <ypu@redhat.com>

qemu/tests/cfg/nfs_perf.cfg

+- nfs_perf:
+    only virtio_net
+    only Linux
+    type = nfs_perf
+    # The following 2 parameters are the nfs server address
+    # and partition, you must modify them before you run
+    # this test. Normally the nfs server is located on 
+    # another machine, but you still can setup a nfs server
+    # on the host which runs the guest.
+    nfs_server = "ip_of_nfs_server"
+    nfs_path = "/path/to/nfs/partition"
+    mnt_option = "rw,bg,hard,nointr,rsize=65536,wsize=65536,tcp,noac,vers=3,timeo=600"
+    blk_size_list = "4k 8k 16k"