You have to change with qemu-img the size of the image first and then change the raspi-config script to make it work.
To resize the raspian image for Qemu:
qemu-img resize 2012-07-15-wheezy-raspbian.img +2G
I add 2G more (2+2=4GB), you can add more. Now you have free space on it so you can add a partition or resize the existing partition
To resize the root filesystem with raspi-config just do this:
http://www.raspberrypi.org/phpBB3/viewt ... 25#p129525
Enter these 4 lines, the last one will start a modified raspi-config script (the original will not be changed), choose resize_rootfs in the menu and it should work now. Reboot.