The normal approach to resizing an ntfs partition (losetup cfdisk,
losetup -o32256 ntfsresize) will not work as you would expect because losetup
and qemu, in the absense of real hardware, will invent disagreeing drive
geometries which Windows' boot sector depends upon to start. It is conceptually
simplest to find a means of running (c|s|)fdisk and ntfsresize from inside qemu
itself.

http://gparted.sourceforge.net/download.php

If you are using a qcow2 image, you probably need to

* qemu-img old.qcow2 new.raw
* truncate -s 100GiB new.raw # or what have you
* qemu-img new.raw -O qcow2 old.qcow2

It is not clear why qcow2 images do not support resizing.