XML: schemas.c: take severities of XSLT messages into account properly

When using native logging as a sink for these messages, just forward
them with the corresponding true severity and with the indicating prefix
stripped. Otherwise, when stderr requested, do some additional gating
per the currently set verbosity so that, for instance, INFO messages
get shown with "cibadmin -V" (i.e., single verbosity upvote; double
then to include also DEBUG messages, which is exactly what's desirable
with "upgrade" subtest of cts-cli suite).