Description of problem:
save memsave file to read only nfs file system via qmp. qmp show error:
{"error": {"class": "GenericError", "desc": "Could not open 'filename'"}}

Tested other qmp command. e.g screendump.  It show a correctly error message.
{"error": {"class": "GenericError", "desc": "failed to open file 'screen-file': Read-only file system"}}

Version-Release number of selected component (if applicable):
qemu-kvm-rhev-1.5.3-10.el7.x86_64
3.10.0-35.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot guest with qmp
/usr/libexec/qemu-kvm -M q35 -cpu SandyBridge -enable-kvm -m 10G -smp 8,sockets=2,cores=4,threads=1,maxcpus=160 -usb -device usb-tablet,id=input0 -name sluo -uuid ec15da0a-e8a7-42f4-905a-78fed78fd8fd -rtc base=localtime,clock=host,driftfix=slew -device ahci,id=ahci0,addr=0x04 -drive file=/home/win8.1-32.qcow2,if=none,id=drive-sata-disk-0-0,format=qcow2 -device ide-drive,bus=ahci0.0,unit=0,drive=drive-sata-disk-0-0,id=sata-disk-0-0 -netdev tap,script=/etc/qemu-ifup,id=netdev0 -device virtio-net-pci,netdev=netdev0,id=virtio-net-pci0,mac=e4:1f:13:eb:40:24,addr=0x5 -device virtio-balloon-pci,id=b1 -vnc :1 -monitor stdio  -boot menu=on -spice port=5931,disable-ticketing -vga qxl -global qxl-vga.revision=3 -qmp tcp:0:4444,server,nowait

2.save memsave file to read only nfs file system via qmp
{"execute":"memsave","arguments":{"val":1,"size":9999999,"filename":"mem-dump"}}

3.

Actual results:
{"error": {"class": "GenericError", "desc": "Could not open 'mem-dump'"}}

Expected results:
{"error": {"class": "GenericError", "desc": "failed to open file 'mem-dump': Read-only file system"}}

Additional info: