virt-libvirt: net_state_dict() use another virsh

This virsh module function becomes a Virsh class method, but
internally it makes other calls to the virsh module directly.
Sometimes this is not what is needed, rather a specific Virsh
class instance should be used (i.e. to run against a remote
system or with special args/dargs).  Added a parameter
with None default to allow this.  Fixed up all references to
this function so they also pass through proper virsh_instance
state.
Signed-off-by: NChris Evich <cevich@redhat.com>