VMware: Improve the efficiency of vm_util.get_host_name_for_vm

get_host_name_for_vm was extremely and unnecessarily inefficient. In
fetching a host, it:

1. Got the host_ref for the VM
2. Threw away the type component of the host_ref
3. Retrieved all HostSystem objects known to vSphere
4. Searched them for the object matching the value component of the
   host_ref
5. Retrieved the name property from the returned component

The new implementation does:

1. Get the host_ref for the VM
2. Get the name property of the HostSystem object with that host_ref

Consequently, the following functions are no longer used, and go away:

* get_host_ref_from_id
* property_from_property_set
* get_host_id_from_vm_ref

get_host_ref_for_vm is modified in both its implementation (see
above), and to return the host_ref its name suggests rather than an
ObjectContent object.

Change-Id: I89a731b2c5df61f031b93c861b2e4a222da49e99
Merge "VMware: Improve the efficiency of vm_util.get_host_name_for_vm"