softmmu/qtest: Move the target-specific pseries RTAS code out of qtest.c

softmmu/qtest: Move the target-specific pseries RTAS code out of qtest.c

Ideally, qtest.c should be independent from target specific code, so
we only have to compile it once for all targets. Thus start improving
the situation by moving the pseries related code to hw/ppc/spapr_rtas.c
instead and allow target code to register a callback handler for such
target specific commands.

Message-Id: <20230411183418.1640500-2-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>