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.

Change-Id: If7c3b14dd49ed05c5fde819c5a36d5608650cbbc
Closes-Bug: #1255609
Merge "VMware: fix the VNC port allocation"