qemu-iotests: 086 doesn't work with NFS

The reference output file only works for file. 'qemu-img convert -p'
makes a lot more progress updates for NFS than for file, so disable the
test for NFS.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>

tests/qemu-iotests/086

@@ -38,7 +38,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.filter
 
 _supported_fmt qcow2 raw
-_supported_proto file nfs
+_supported_proto file
 _supported_os Linux
 
 function run_qemu_img()