ruby: change value of 'readonly' drive toption to Boolean in doc/example/test

Seeing `g.add_drive_opt :readonly => 1` allows one to imply
that ensuring writable access to drive should happen via
`g.add_drive_opt :readonly => 0`. However, the passed option
value gets passed down to C according to Ruby Boolean semantics,
that is, any value apart from `false` and `nil` will be true
(see RTEST in Ruby C API).

So its more idiomatic and provides a better hint if we use
`g.add_drive_opt :readonly => true` in Ruby samples.

ruby/examples/guestfs-ruby.pod

@@ -7,7 +7,7 @@ guestfs-ruby - How to use libguestfs from Ruby
  require 'guestfs'
  g = Guestfs::Guestfs.new()
  g.add_drive_opts("disk.img",
-                  :readonly => 1, :format => "raw")
+                  :readonly => true, :format => "raw")
  g.launch()
 
 =head1 DESCRIPTION

ruby/examples/inspect_vm.rb

@@ -11,7 +11,7 @@ disk = ARGV[0]
 g = Guestfs::Guestfs.new()
 
 # Attach the disk image read-only to libguestfs.
-g.add_drive_opts(disk, :readonly => 1)
+g.add_drive_opts(disk, :readonly => true)
 
 # Run the libguestfs back-end.
 g.launch()

ruby/t/tc_070_optargs.rb

@@ -22,9 +22,9 @@ class Test070Optargs < MiniTest::Unit::TestCase
     g = Guestfs::Guestfs.new()
 
     g.add_drive("/dev/null", {})
-    g.add_drive("/dev/null", :readonly => 1)
-    g.add_drive("/dev/null", :readonly => 1, :iface => "virtio")
+    g.add_drive("/dev/null", :readonly => true)
+    g.add_drive("/dev/null", :readonly => true, :iface => "virtio")
     g.add_drive("/dev/null",
-                :readonly => 1, :iface => "virtio", :format => "raw")
+                :readonly => true, :iface => "virtio", :format => "raw")
   end
 end