Use the method respond_to? to check if a method exists before calling it. See also the libguestfs availability API for larger features.
$ irb irb(main):001:0> require 'guestfs' => true irb(main):002:0> g = Guestfs::create() => #<Guestfs::Guestfs:0x7f7b0d88c450> irb(main):003:0> g.respond_to?( 'set_network' ) => true irb(main):004:0> g.respond_to?( 'no_such_method' ) => false