Hi Deepak,
The VM harddisk quantity is a bit buried. You can find the storage for a particular harddisk of a particular VM like so:
$vm=get-civm
$hardware=$vm.ExtensionData.GetVirtualHardwareSection()
($hardware.Item |where {$_.resourcetype.value -eq"17"}).hostresource[0].anyattr[0]."#text"
If you are querying more than 1 VM, or a VM with more than 1 harddisk, you will have to modify this accordingly.
Cheers,
Jake