in podman machine, we call gvproxy by the command line in at least three different locations in machine.  creating lengthy command lines in podman for gvproxy is error prone and requires maintenance if upstream gvproxy changes.

following the lead of vfkit, we should create a struct that describes gvproxy.  then the struct should be able to be converted into a consistent and valid gvproxy command line.  perhaps even the exec command could be a return value.