VMware: Skip vSAN for preallocated image download

VMware: Skip vSAN for preallocated image download

Copying preallocated image to volume involves creating
a virtual disk using the image. The virtual disk has a
descriptor and an extent. The preallocated image is used
as the flat extent to create the virtual disk.

To create virtual disk from preallocated image, we first
create a temporary virtual disk (with a descriptor and
flat extent), then replace its flat extent with the
downloaded image. Since vSAN datastores do not support
flat extent, the above operation fails if such a datastore
is selected for virtual disk creation. This patch fixes
the problem by not selecting vSAN datastores for temporary
virtual disk creation.

Change-Id: Ib4c0fcf5c379b137e70378f137d996bf510f05b3
Closes-Bug: #1301943