I'm currently doing the following process on the Host
mv vm.qcow2 vm.qcow2.bak qemu-img create -f qcow2 addon.qcow2 +100G cat vm.qcow2.bak addon.qcow2 >> vm.qcow2
This works, but when I book up the VM it still shows the old hard drive size?
The sequence of commands you've used results in a new image file with the original, un-resized content, and some new content is appended but entirely inaccessible.
In order to resize a QCOW2 disk image, you should use
qemu-img resize instead. For example:
qemu-img resize vm.qcow2 +100G
There's no need to
Keep in mind that this only changes the size of the virtual disk; you still need to use partitioning and filesystem tools to resize the virtual disk partitions and the filesystems contained on the virtual disk.