Set rsync_command for Haiku guests

At the time of writing, the Haiku rsync lacks old-style --compress due
to its external zlib. Pass `-zz` to the guest rsync.

This works with or without the `--compress` flag in the host's
`rsync__args`.

plugins/guests/haiku/cap/rsync.rb

@@ -9,6 +9,10 @@ module VagrantPlugins
         def self.rsync_install(machine)
           machine.communicate.execute("pkgman install -y rsync")
         end
+
+        def self.rsync_command(machine)
+          "rsync -zz"
+        end
       end
     end
   end

plugins/guests/haiku/plugin.rb

@@ -40,6 +40,11 @@ module VagrantPlugins
         require_relative "cap/rsync"
         Cap::RSync
       end
+
+      guest_capability(:haiku, :rsync_command) do
+        require_relative "cap/rsync"
+        Cap::RSync
+      end
     end
   end
 end

test/unit/plugins/guests/haiku/cap/rsync_test.rb

@@ -35,4 +35,12 @@ describe "VagrantPlugins::GuestHaiku::Cap::RSync" do
       cap.rsync_installed(machine)
     end
   end
+
+  describe ".rsync_command" do
+    let(:cap) { caps.get(:rsync_command) }
+
+    it "defaults to 'rsync -zz'" do
+      expect(cap.rsync_command(machine)).to eq("rsync -zz")
+    end
+  end
 end