Login to your host and do the following:

1. List the qcow2 files from your VM

   virsh domblklist myVM

2. Check the current size from the file

   qemu-img info /var/lib/libvirt/images/myVM.qcow2

3. Resize the the disk file

   qemu-img resize /var/lib/libvirt/images/myVM.qcow2 +16G

It is not required to check the current disk size, but it is good to know. You can take a look afterwards if the resize worked.