shared: introduce MachineRegistrationContext to track bus and registration state

shared: introduce MachineRegistrationContext to track bus and registration state

Bundle scope, buses, and registration success booleans into a
MachineRegistrationContext struct. This eliminates the reterr_registered_system and
reterr_registered_user output parameters from
register_machine_with_fallback_and_log() and the corresponding input
parameters from unregister_machine_with_fallback_and_log().

The struct carries state from registration to unregistration so the
caller no longer needs to manually thread individual booleans between
the two calls.

register_machine_with_fallback_and_log() goes from 7 to 3 parameters,
unregister_machine_with_fallback_and_log() goes from 5 to 2.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>