qemu-log: Add new log category for guest bugs

Add a new category for device models to log guest behaviour
which is likely to be a guest bug of some kind (accessing
nonexistent registers, reading 32 bit wide registers with
a byte access, etc). Making this its own log category allows
those who care (mostly guest OS authors) to see the complaints
without bothering most users.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>