Description of problem:
insufficient disk space during guest installation when use qcow2 img which created by qemu-img

Version-Release number of selected component (if applicable):
python-virtinst-0.500.5-3.el6.noarch

How reproducible:
always

Steps to Reproduce:
1.create qcow2 image using qemu-img
#qemu-img create /tmp/test.img 4G -f  qcow2
 Formatting '/tmp/test.img', fmt=qcow2 size=4294967296 encryption=off cluster_size=0
2:# qemu-img info test.img
image: test.img
file format: qcow2
virtual size: 4.0G (4294967296 bytes)
disk size: 136K
cluster_size: 65536

2.use virt-install to install a guest
#virt-install -n test -r 1024 --hvm --accelerate -c /tmp/img/rhel61.iso --disk path=/tmp/test.img,format=qcow2 
3.during guest installation:
 Which type of installation would you like?
select "Use All Space" or "Replace Existing Linux System" and continue
 after select server type:
Actual results:
 an error dialog shown up
insufficient disk space
You need more space on the following file systems:
93M on /
only occur on 4G img file, 6G and 8G can worked well.
Expected results:
installation continue

Additional info:
if select create custom layout,and create partition manually, install will succeed.