target/ppc: improve checkstop logging

target/ppc: improve checkstop logging

Change the logging not to print to stderr as well, because a
checkstop is a guest error (or perhaps a simulated machine error)
rather than a QEMU error, so send it to the log.

Update the checkstop message, and log CPU registers too.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Glenn Miles <milesg@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>