Description of problem:
The same scenario as in https://bugzilla.redhat.com/show_bug.cgi?id=1236758. 
Tried to do live deletion of a snapshot after the VM disk was extended. The operation ends with a failure in VDSM:

acff4189-cfc2-404d-9c03-ddfb33e36104::ERROR::2015-08-16 10:45:58,925::task::866::Storage.TaskManager.Task::(_setError) Task=`acff4189-cfc2-404d-9c03-ddfb33e36104`::Unexpected error
Traceback (most recent call last):
  File "/usr/share/vdsm/storage/task.py", line 873, in _run
    return fn(*args, **kargs)
  File "/usr/share/vdsm/storage/task.py", line 332, in run
    return self.cmd(*self.argslist, **self.argsdict)
  File "/usr/share/vdsm/storage/securable.py", line 77, in wrapper
    return method(self, *args, **kwargs)
  File "/usr/share/vdsm/storage/sp.py", line 1304, in extendVolumeSize
    .produceVolume(imgUUID, volUUID).extendSize(int(newSize))
  File "/usr/share/vdsm/storage/volume.py", line 552, in extendSize
    raise se.VolumeNonWritable(self.volUUID)
VolumeNonWritable: Volume cannot be access to writes: ["u'46d93a59-0ec1-4c7c-9f25-816be8d0760b'"]


Version-Release number of selected component (if applicable):
ovirt-3.6.0-5
vdsm-4.17.0-1239.git6575e3f.el7.noarch
libvirt-daemon-1.2.8-16.el7_1.3.x86_64
qemu-kvm-ev-2.1.2-23.el7_1.6.1.x86_64
sanlock-3.2.2-2.el7.x86_64
selinux-policy-3.13.1-23.el7_1.13.noarch
python-2.7.5-18.el7_1.1.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Create a VM with 1gb preallocated disk (probably thin as well, didn't test that) in a block-based domain.
2. Create a snapshot.
3. Extend the disk to 2gb.
4. Start the VM.
5. Delete the snapshot.


Actual results:
Live snapshot deletion fails on VDSM with the mentioned exception

Expected results:
Live merge should succeed for a snapshot that was previously extended.

Additional info:
sosreport from host: http://file.tlv.redhat.com/ebenahar/sosreport-green-vdsc.qa.lab.tlv.redhat.com-20150816120535.tar.xz
sosreport from engine: http://file.tlv.redhat.com/ebenahar/sosreport-RHEL6.7Server-20150816120920.tar.xz