Got this from here: http://blogs.technet.com/b/danstolts/archive/2011/04/02/how-to-expand-and-extend-to-increase-capacity-on-a-virtual-hard-disk-the-easy-and-fast-way-solve-the-problem-of-out-of-disk-space-on-drive-c-forever.aspx
First, if you have subversions of the PVS disk, merge it to a new base and then rename the new vhd, pvp and lok file. Delete the merged version from the PVS console.
Note, this is not a script, manually run each task, be careful to select correct disk and volume!
diskpart Select vdisk file="E:storemynew.vhd" list vdisk expand vdisk maximum=50000 (in megabytes) attach vdisk list disk list volume select volume 7 (volume number from list volume command) extend list volume detach vdisk exit
Import the new disk in the PVS console.