Fix issue where we may not get the full output in a subprocess

lib/vagrant/util/subprocess.rb

@@ -65,9 +65,23 @@ module Vagrant
 
           # Check the readers to see if they're ready
           if !readers.empty?
-            begin
             readers.each do |r|
-                data = r.read_nonblock(1024)
+              data = ""
+              while true
+                begin
+                  data << r.read_nonblock(1024)
+                rescue IO::WaitReadable
+                  # This just means the IO wasn't actually ready and we
+                  # should wait some more. No problem! Just pass on through...
+                rescue EOFError
+                  # Process exited, most likely. We're done here.
+                  break
+                end
+              end
+
+              # We don't need to do anything if the data is empty
+              next if data.empty?
+
               io_name = r == stdout ? :stdout : :stderr
               @logger.debug(data)
 
@@ -80,13 +94,6 @@ module Vagrant
               io_data[r] += data
               yield io_name, data if block_given?
             end
-            rescue IO::WaitReadable
-              # This just means the IO wasn't actually ready and we
-              # should wait some more. No problem! Just pass on through...
-            rescue EOFError
-              # Process exited, most likely. We're done here.
-              break
-            end
           end
 
           # Break out if the process exited. We have to do this before