VMware: Fix datastore selection with single host

VMware: Fix datastore selection with single host

Currently, the select_datastore method is called with an empty list of
ESX hosts. This causes the method to retrieve all the hosts in vCenter
as candidates for selecting a datastore. The reference to a retrieved
host is stored in attribute 'obj'. Some of the future bug fixes will
need to invoke this method with reference to a single ESX host. Such
invocations will fail with attribute error: 'val instance has no
attribute 'obj''. This patch fixes the attribute error.

Closes-Bug: #1401052
Change-Id: Ib4f48d2ebfbfa3a9ce6a402300d41351548932ef