Add additional debug info for mac searching

Reporting the MAC address being looked for in the ARP table should help
determine why some machines pick up an address belonging to another
when starting multiple VM's in parallel.

lib/vagrant-libvirt/action/wait_till_up.rb

@@ -33,6 +33,7 @@ module VagrantPlugins
           # connection was done via ssh.
           env[:ip_address] = nil
           env[:metrics]["instance_ip_time"] = Util::Timer.time do
+            @logger.debug("Searching for IP for MAC address: #{domain.mac}")
             env[:ui].info(I18n.t("vagrant_libvirt.waiting_for_ip"))
             retryable(:on => Fog::Errors::TimeoutError, :tries => 300) do
               # If we're interrupted don't worry about waiting

spec/support/libvirt_context.rb

@@ -21,5 +21,8 @@ shared_context "libvirt" do
     # drivers also call vm_exists? during init;
     allow(connection).to receive(:servers).with(kind_of(String)).
       and_return(connection_result(result: nil))
+
+    # return some information for domain when needed
+    allow(domain).to receive(:mac).and_return("9C:D5:53:F1:5A:E7")
   end
 end

spec/unit/action/wait_till_up_spec.rb

@@ -58,6 +58,7 @@ describe VagrantPlugins::ProviderLibvirt::Action::WaitTillUp do
           expect(ui).to receive(:info).with("Waiting for domain to get an IP address...")
           expect(ui).to receive(:info).with("Waiting for SSH to become available...")
           logger = subject.instance_variable_get(:@logger)
+          expect(logger).to receive(:debug).with(/Searching for IP for MAC address: .*/)
           expect(logger).to receive(:info).with("Got IP address 192.168.121.2")
           expect(logger).to receive(:info).with(/Time for getting IP: .*/)
           expect(env[:machine].communicate).to_not receive(:ready?)