hw/display/bcm2835_fb: Reset resolution, etc correctly

hw/display/bcm2835_fb: Reset resolution, etc correctly

The bcm2835_fb's initial resolution and other parameters are set
via QOM properties. We should reset to those initial values on
device reset, which means we need to save the QOM property
values somewhere that they are not overwritten by guest
changes to the framebuffer configuration.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180814144436.679-5-peter.maydell@linaro.org