refactor net option
    
    Currently net options means either nfs or ssh dump.
    Better to split these two into standalone options. That's more clear to user.
    
    after the split, ssh dump need user specify "ssh user@host"
    nfs dump need user specify "nfs host:nfsshare"
    
    Signed-off-by: Dave Young <dyoung@redhat.com>
    Acked-by: Vivek Goyal <vgoyal@redhat.com>