hw/misc: applesmc: use host osk as default on macs

hw/misc: applesmc: use host osk as default on macs

When running on a Mac, QEMU is able to get the host OSK and use it as
the default value for the AppleSMC device. The OSK query operation
doesn't require administrator privileges and can be executed by any user
on the system. This patch is based on open-source code from Apple, just
like the implementation from VirtualBox.

Apple:
https://opensource.apple.com/source/IOKitUser/IOKitUser-647.6.13/pwr_mgt.subproj/IOPMLibPrivate.c
https://opensource.apple.com/source/PowerManagement/PowerManagement-637.60.1/pmconfigd/PrivateLib.c

VirtualBox:
https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Devices/EFI/DevSmc.cpp#L516

Signed-off-by: Pedro Tôrres <t0rr3sp3dr0@gmail.com>