VMWare: Fix nova-compute crash when instance datastore not available

During nova-compute service startup, init_host will try to get_info for
instances. If datastore of certain instances become not reachable, some
VM properties like 'summary.config.memorySizeMB' will not be available.
This will cause nova-compute process exit with error.

Change-Id: I6753ce5658b629c216ebfbaab610c8fd0755466a
Closes-Bug: #1337236
Merge "VMWare: Fix nova-compute crash when instance datastore not available"