Add support for QEMU

[bifurcation]

QEMU is commonly used for emulating embedded devices, e.g., in the exercises for the Rust Embedded Book. It looks like QEMU will not be able to effectively emulate all the peripherals we have attached, but it might be able to provide some utility.

For example, the cmox crate has pretty minimal dependency on the features of the UI chip, beyond core peripherals provided by the chip. It might be possible to use QEMU to run cmox tests on a laptop or a Github CI runner. Or it might be possible to do some basic testing of the ui-stm32 framework, if say you could detect that the firmware had set a given GPIO pin to a given value.

[bifurcation]

An alternative might be the Renode framework, which seems to have pretty good stm32f4 support, and explicitly supports multi-node systems such as we have here.