If you're using qcow VM images and need to expand storage capacity it is very easy. Simply shutdown the VM, and run the following command on the VM image file.

qemu-img resize /mnt/server/server.qcow +10G

This will add an additional 10GB to the image. Next, boot the VM back up and resize the partitions to use the newly added space. As your / partition is usually last you can just delete your existing partition, and immediately create a new partition with the added 10GB of space. Then you can use resize2fs for ext3/ext4 file systems, or xfs_growfs for xfs file systems to expand the partition to file the added space.

Borrowed from positon.org.