vcsim: use VirtualDisk CapacityInKB for device summary
vcsim: use VirtualDisk CapacityInKB for device summary

Fixes #1108

bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,dougm/govmomi,dougm/govmomi,dougm/govmomi,bruceadowns/govmomi,vmware/govmomi