datapath-windows: Add functionality to return OVSPortName given a VM

This commit adds functionality to return the ElementName of all the VNIC of a
given virtual machine name.

The function returns all the allocated ports including the ones which the OVS
port name was not set.

The default OVS port name that is set is the following:
"ElementName             : Dynamic Ethernet Switch Port".

Example of usage:
Import-Module OVS.psm1
Get-VMNetworkAdapterWithOvsPort test_ubuntu_1
$a = Get-VMNetworkAdapterWithOvsPort test_2_1
$a | findstr ElementName

Requested-by: Nithin Raju <nithin@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/51
Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Tested-by: Nithin Raju <nithin@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>