Fix foreign key constraint error on ml2_dvr_port_bindings

Get the port before creating the DVR binding. This should guarantee that, if
the port is found, no foreign key violation is triggered.

Closes-bug: #1371696

Change-Id: I90c3d6460c14ef27823be95bc26aae38eba4879a
Merge "Fix foreign key constraint error on ml2_dvr_port_bindings"