Also met this issue in rhe6.5 host.

host info:
# uname -r && rpm -q qemu-kvm-rhev 
2.6.32-414.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.400.el6.x86_64

# qemu-img convert -p -f vhdx sluo-win2012-64bit.vhdx -O qcow2  sluo-win2012-64bit.qcow2
    (98.25/100%)
# qemu-img check sluo-win2012-64bit.qcow2
No errors were found on the image.
Image end offset: 8431927296

Aslo tried the rhel6.4GA that also this issue, so it's not a regression issue.

host info:
# rpm -q qemu-kvm
qemu-kvm-0.12.1.2-2.355.el6_4.7.x86_64

Steps and Results:
# qemu-img info win7-64.qcow2
image: win7-64.qcow2
file format: qcow2
virtual size: 30G (32212254720 bytes)
disk size: 8.1G
cluster_size: 65536
# qemu-img convert -p -f qcow2 win7-64.qcow2 -O raw win7-64.raw
    (98.25/100%)

Best Regards,
sluo

Fix included in qemu-kvm-0.12.1.2-2.456.el6

Reproduce this bug with qemu-kvm-0.12.1.2-2.454.el6.x86_64

# qemu-img convert -f qcow2 rhel6.7cp1.qcow2 -O raw rhel6.7_repro.raw -p
    (98.25/100%)


Verify this bug with qemu-kvm-0.12.1.2-2.457.el6.x86_64

# qemu-img convert -f qcow2 rhel6.7cp1.qcow2 -O raw rhel6.7.raw -p
    (100.00/100%)


So this bug is fixed.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-1275.html