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.