Fixes host stats for VMWareVCDriver

Host stats for VCDriver should collect aggregate cluster stats
rather than that of a single host in the cluster.

Fixes: bug #1190515
Change-Id: I37e46995c5da2e3052e8178098afee7c8061bb3c

Conflicts:

	nova/tests/virt/vmwareapi/test_vmwareapi.py
	nova/virt/vmwareapi/driver.py