Description of problem:
As subject, If guest image is on NFS, discard will be invalid.

Version-Release number of selected component (if applicable):
# uname -r
3.10.0-112.el7.x86_64
qemu-kvm-rhev-1.5.3-53.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot guest and attached a data disk on NFS(/mnt/ss.raw) and enable discard

/usr/libexec/qemu-kvm -M pc -cpu Opteron_G1 -device virtio-scsi-pci,id=scsi2,indirect_desc=off,event_idx=off,bus=pci.0,addr=0x8 -drive file=/mnt/ss.raw,if=none,id=drive-hd-disk-3,media=disk,format=raw,cache=none,werror=stop,rerror=stop,discard=on -device scsi-hd,drive=drive-hd-disk-3,id=scsi_disk3 -vnc :1  -drive file=/mnt/RHEL-Server-7.0-64.qcow2,if=none,id=drive-virtio-disk,format=qcow2,cache=none,aio=native,werror=stop,rerror=stop,media=disk,discard=on,serial,copy-on-read=on,snapshot=on -device virtio-blk-pci,vectors=0,scsi=off,drive=drive-virtio-disk,id=virtio-disk,bus=pci.0,addr=0x7,bootindex=1

2.format data disk inside guest with ext4
#mkfs.ext4 /dev/sda
#mount /dev/sda /mnt
#dd if=/dev/urandom of=/mnt/test 

3.check data disk image in host
# qemu-img map --output=json /mnt/ss.raw 

Actual results:
{ "start": 0, "length": 20971520, "depth": 0, "zero": false, "data": true, "offset": 0}]

Expected results:
discard valib

Additional info:
If data disk image is on local(not NFS), then discard works well