Description of problem:
make a vmdk format image, then create first external snapshot1 with the backing file, then create second external snapshot2 with snapshot1 as backing file. but it fail to crate snapshot2 with prompt 'No such file or directory'. btw, qcow2 have no such issue.
e.g: base<-----snap1<-------snap2<----...

Version-Release number of selected component (if applicable):
host info:
2.6.32-392.el6.x86_64
qemu-kvm-0.12.1.2-2.376.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1.create a vmdk format disk.
# qemu-img create -f vmdk vmdk-data-disk.vmdk 10G
Formatting 'vmdk-data-disk.vmdk', fmt=vmdk size=10737418240 compat6=off zeroed_grain=off 
# qemu-img info vmdk-data-disk.vmdk 
image: vmdk-data-disk.vmdk
file format: vmdk
virtual size: 10G (10737418240 bytes)
disk size: 12K
2.create first external snapshot1 with the backing file.
# qemu-img create -f vmdk -b vmdk-data-disk.vmdk vmdk-data-snapshot1
Formatting 'vmdk-data-snapshot1', fmt=vmdk size=10737418240 backing_file='vmdk-data-disk.vmdk' compat6=off zeroed_grain=off 
# qemu-img info vmdk-data-snapshot1
image: vmdk-data-snapshot1
file format: vmdk
virtual size: 10G (10737418240 bytes)
disk size: 12K
backing file: disk.vmdk
# ls -lh vmdk-data-snapshot1
-rw-r--r--. 1 root root 1.4M Jun 25 17:56 vmdk-data-snapshot1
3.create second external snapshot3 with the backing file.
# qemu-img create -f vmdk -b vmdk-data-snapshot1 vmdk-data-snapshot2

Actual results:
after step 3, it fail to crate snapshot2 with prompt 'No such file or directory'.
# qemu-img create -f vmdk -b vmdk-data-snapshot1 vmdk-data-snapshot2
Could not open 'vmdk-data-snapshot1': No such file or directory

Expected results:
it should can ordinal create external snapshot with vmdk format one by one. 

Additional info: