Description of problem:
After one of snapshot chain images rewrite, can not use snapshot_blkdev to create new snapshot. 
When rewrite one of snapshot chain images, qmp does not give any hint.

Version-Release number of selected component (if applicable):
qemu-kvm-rhev-1.5.3-19.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot guest.
<cli>:
# /usr/libexec/qemu-kvm -M pc-i440fx-rhel7.0.0 -cpu SandyBridge -enable-kvm -m 4G -smp 4,sockets=2,cores=2,threads=1 -name juli -uuid 355a2475-4e03-4cdd-bf7b-5d6a59edaa68 -rtc base=localtime,clock=host,driftfix=slew \
-device virtio-scsi-pci,bus=pci.0,addr=0x5,id=scsi0 -drive file=iscsi://10.66.6.82:3260/iqn.2013-11.com.example:storage.disk1.juli.xyz/1,if=none,id=drive-scsi0-0-0,media=disk,cache=none,format=qcow2,werror=stop,rerror=stop,aio=native  -device scsi-hd,drive=drive-scsi0-0-0,bus=scsi0.0,scsi-id=0,lun=0,id=juli,bootindex=2 \
-drive file=/home/ISO/en_windows_8.1_preview_x86_dvd_2358833.iso,if=none,media=cdrom,format=raw,aio=native,id=drive-ide1-0-0 -device ide-drive,drive=drive-ide1-0-0,id=ide1-0-0,bus=ide.0,unit=0,bootindex=4 -device virtio-balloon-pci,id=ballooning -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 -net none -k en-us -boot menu=on,reboot-timeout=-1,strict=on -qmp tcp:0:4477,server,nowait -serial unix:/tmp/ttyS0,server,nowait -vnc :3 -spice port=5939,disable-ticketing  -vga qxl -global qxl-vga.revision=3 -monitor stdio -monitor tcp:0:7777,server,nowait -monitor unix:/tmp/monitor1,server,nowait \
-netdev tap,id=tap1,vhost=on,queues=4,script=/etc/qemu-ifup,downscript=/etc/qemu-ifdown,ifname=tap-juli \
-device virtio-net-pci,netdev=tap1,id=nic1,mq=on,vectors=17,mac=1a:59:0a:4b:aa:94
2.save live snapshot snapshot-n (n=1,2,3) via qmp.
$ telnet 10.66.106.4 4477
{"execute":"qmp_capabilities"}
{"return": {}}
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-1.img", "format": "qcow2" } }
{"return": {}}
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-2.img", "format": "qcow2" } }
{"return": {}}
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-3.img", "format": "qcow2" } }
{"return": {}}
3.save live snapshot snapshot-1 after step 2.
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-1.img", "format": "qcow2" } }
{"return": {}}
4.save live snapshot snapshot-4 via qmp.
 { "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-4.img", "format": "qcow2" } }

Actual results:
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-4.img", "format": "qcow2" } }
{"error": {"class": "GenericError", "desc": "Could not open '/home/snapshot-1.img': Could not open file: Too many open files: Too many open files"}}


Expected results:
When rewrite one of snapshot chain images, qmp give an error message or forbid this behaviour.

Additional info:
After step 2, do the following live snapshot:
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive-scsi0-0-0", "snapshot-file": "/home/snapshot-3.img", "format": "qcow2" } }
{"error": {"class": "GenericError", "desc": "Error: Trying to create an image with the same filename as the backing file"}}

As the above shows, qmp will give an error message.