remove 'filename' from ROM sesion args

Conveying the ROM filename as the final label element simplifies
routing policy and session construction.

Annotations by nfeske:

This commit also changes the ROM session to use base/log.h instead of
base/printf.h, which produced build error of VirtualBox because the
vbox headers have a '#define Log', which collides with the content of
base/log.h. Hence, this commit has to take precautions to resolve this
conflict.

The commit alse refines the previous session-label change by adding a
new 'Session_label::prefix' method and removing the use of 'char const *'
from this part of the API.

Fixes #1787

repos/demo/src/lib/launchpad/launchpad.cc

@@ -217,7 +217,8 @@ Launchpad_child *Launchpad::start_child(const char *filename,
 		 * constructor of 'Rom_connection' throws a 'Parent::Service_denied'
 		 * exception.
 		 */
-		Rom_connection rom(filename, unique_name);
+		Rom_connection rom(prefixed_label(Session_label(unique_name),
+		                                  Session_label(filename)).string());
 		rom.on_destruction(Rom_connection::KEEP_OPEN);
 		rom_cap  = rom.cap();
 		file_cap = rom.dataspace();