util: clean up #includes of virnetdevopenvswitch.h

util: clean up #includes of virnetdevopenvswitch.h

virnetdevopenvswitch.h declares a few functions that can be called to
add ports to and remove them from OVS bridges, and retrieve the
migration data for a port. It does not contain any data definitions
that are used by domain_conf.h. But for some reason, domain_conf.h
virnetdevopenvswitch.h should be directly #including it. This adds a
few lines to the project, but saves all the files that don't need it
from the extra computing, and makes the dependencies more clear cut.