VMware: fix the VNC port allocation

There is small chance for VNC port collisions with the current
implementation which choose the port number based on the MoRef id
of the VM.
This patch fixes this by running a query for all allocated ports
and then selects one which is not taken.

Closes-Bug: #1255609
(cherry picked from commit 2f49ed4b5dbb5c954fc7a9b42ee7b170c38c775c)

Conflicts:

	nova/exception.py
	nova/tests/virt/vmwareapi/test_vmwareapi.py
	nova/tests/virt/vmwareapi/test_vmwareapi_vm_util.py
	nova/virt/vmwareapi/vm_util.py
	nova/virt/vmwareapi/vmops.py

Change-Id: If7c3b14dd49ed05c5fde819c5a36d5608650cbbc
Merge "VMware: fix the VNC port allocation" into stable/havana