VMWare Driver - Ignore datastore in maintenance mode

A datastore can be part of a datastore cluster. Any
datastore which is part of a cluster can be put in maintenance
mode. Instances cannot reside on a datastore if it is in
maintenance mode.
This fix ignores a datastore in the following scenarios
1. Stats update - Ignore a datastore during stats update.
2. Ignore a datastore during instance spawn.

Change-Id: Id0cb3f5fdb4d0bc4884cf5405fadc433d4a2b6ba
Closes-Bug: #1330065
Merge "VMWare Driver - Ignore datastore in maintenance mode"