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
2b8c1af9e0