Fixed in qemu-kvm-0.12.1.2-2.415.el6_5_5.3

Verify this bug with component:
qemu-img-0.12.1.2-2.415.el6_5.3.x86_64

Step:
1. Create an image with 'cluster_size=512,preallocation=metadata', and then query the information.
# time qemu-img create -f qcow2 -o cluster_size=512,preallocation=metadata disk.img 4G; time qemu-img info disk.img

Result:
After step1,
Formatting 'disk.img', fmt=qcow2 size=4294967296 encryption=off cluster_size=512 preallocation='metadata' 

real	5m52.712s
user	0m4.461s
sys	0m11.529s
image: disk.img
file format: qcow2
virtual size: 4.0G (4294967296 bytes)
disk size: 530M
cluster_size: 512

real	0m0.006s
user	0m0.000s
sys	0m0.004s


Also tested with default cluster_size and 'preallocation=metadata', the resule shows below.
# time qemu-img create -f qcow2 -o preallocation=metadata disk.img 4G; time qemu-img info disk.img
Formatting 'disk.img', fmt=qcow2 size=4294967296 encryption=off cluster_size=65536 preallocation='metadata' 

real	0m0.367s
user	0m0.005s
sys	0m0.207s
image: disk.img
file format: qcow2
virtual size: 4.0G (4294967296 bytes)
disk size: 784K
cluster_size: 65536

real	0m0.004s
user	0m0.001s
sys	0m0.002s


Base on these test results above, this bug has been 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.

http://rhn.redhat.com/errata/RHBA-2013-1750.html