VMware: Detach volume should not delete vmdk

During volume detach a delete_virtual_disk_spec is used to remove
the device from the running instance. This spec also "destroy" the
underlying vmdk file, this is not right, we should not delete vmdk
file when detach it from a VM instance.

The fix was remove the fileOperation field when detach vmdk volume
so as to make sure the volume will not be destroyed; but for iscsi
we still need to destroy the volume.

Also I changed the function name from delete_virtual_disk_spec to
detach_virtual_disk_spec which is more accurate.

Change-Id: Ibd218d6a8cfeede4f5ca74f28bc0c3d0c185bb14
Closes-Bug: #1241350
Merge "VMware: Detach volume should not delete vmdk"