Shutdown/Reboot - Githubissues

Summary

Implemented shutdown functionality

Partial fix for #38, only shutdown for now

Added implementation for ACPI to read the relevant data \_Sx from AML to be prepared for shutdown/sleep.
Added implementation to use the \_Sx data to issue a shutdown/sleep with ACPI.
Implemented unmounting for filesystems (for now we can't unmount a specific fs, but all at once as part of the shutdown process).
Implemented shutdown process which is as follows:
- power::start_shutdown is called.
- scheduler stops scheduling and starts exiting processes when they arrive for rescheduling, keeps waiting until all have exited.
- the filesystem is unmounted and any related data is flushed.
- Issue a shutdown command to ACPI to finally shutdown the machine.
Implement reboot process which is similar to the above shutdown process except the last part:
- Issue a reset command through the PS2 interface in the keyboard/mouse device.
- We can implement this in ACPI using the reset_register, but its not available for my testing setup now.
Userspace access to shutdown and reboot through /devices/power virtual device
- shutdown can be started by echo shutdown > /devices/power or echo reboot > ...
Added power commands that does the above, choose shutdown or reboot

[x] The changes are tested and works as expected (mention if not)
[ ] Tests if applicable (new features, regression tests, etc...)
- won't do this for now, I want to write test but it would be tricky, since we need a custom test image that does shutdown and reboot and a userspace (linux program/shell) that monitors qemu and asserts shutdown/reboot is good.
[x] Documentation
[ ] Needed README changes