Fix session guid handling in python 3

I made the mistake of using str.decode() which isn't a thing in python3
(works fine in 2). So this commit fixes it so that the GUID string
itself is generated directly as a byte string, so that the call to
decode() isn't needed at all.

Author: OJ

lib/msf/core/payload/python/meterpreter_loader.rb

@@ -75,11 +75,11 @@ def stage_meterpreter(opts={})
     met.sub!("PAYLOAD_UUID = \'\'", "PAYLOAD_UUID = \'#{uuid}\'")
 
     if opts[:stageless] == true
-      session_guid = "00" * 16
+      session_guid = '\x00' * 16
     else
-      session_guid = SecureRandom.uuid.gsub(/-/, '')
+      session_guid = SecureRandom.uuid.gsub(/-/, '').gsub(/(..)/, '\\x\1')
     end
-    met.sub!("SESSION_GUID = \'\'", "SESSION_GUID = \'#{session_guid}\'.decode(\'hex\')")
+    met.sub!("SESSION_GUID = \'\'", "SESSION_GUID = \'#{session_guid}\'")
 
     http_user_agent = opts[:http_user_agent] || ds['MeterpreterUserAgent']
     http_proxy_host = opts[:http_proxy_host] || ds['PayloadProxyHost'] || ds['PROXYHOST']