Migration always uses a TCP socket for NBD servers, because we don't
support same-host migration. But upcoming pull-mode incremental backup
needs to also support a Unix socket, for retrieving the backup from
the same host. Support this by plumbing virStorageNetHostDef through
the monitor calls, since that is a nice reusable struct that can track
both TCP and Unix sockets.

Update qemumonitorjsontest to verify both forms of the QMP command.
Signed-off-by: NEric Blake <eblake@redhat.com>
Acked-by: NPeter Krempa <pkrempa@redhat.com>