The previous approach of traversing the machine and its device tree
at startup to register CLI11 options was not working for dynamically
registered devices like PCI cards. This meant that options like
gfxmem_size or mon_id from the video cards could not be set.
Switch to instead registering in MachineFactory a hook function that
provides CLI flag values. We can call it when registering any property,
whether at startup or dynamically.
Result of running IWYU (https://include-what-you-use.org/) and
applying most of the suggestions about unncessary includes and
forward declarations.
Was motivated by observing that <thread> was being included in
ppcopcodes.cpp even though it was unused (found while researching
the use of threads), but seems generally good to help with build
times and correctness.
