datapath-windows: Rename 'vport->isPresentOnHv' to 'isAbsentOnHv'

datapath-windows: Rename 'vport->isPresentOnHv' to 'isAbsentOnHv'

Looking at the code, the flag 'vport->isPresentOnHv' is actually
indicating if the vport is present on the Hyper-V switch or not, but the
logic seems to be inverse. 'isPresentOnHv == TRUE' indicates that the
vport is not present on the Hyper-V switch. Eg. VXLAN port, would have
isPresentOnHv == TRUE.

In this patch, we rename the variable to reflect its meaning.

vport->isAbsentOnHv is TRUE iff:
- vport is bridge internal port
- vport is tunnel port
- vport was added from Hyper-V and also from OVS, but got deleted from
Hyper-V

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>