machine_class_base_init() member name is allocated by
machine_class_base_init(), but not freed by
machine_class_finalize(). Simply freeing there doesn't work,
because DEFINE_PC_MACHINE() overwrites it with a literal string.
Fix DEFINE_PC_MACHINE() not to overwrite it, and add the missing
free to machine_class_finalize().
Backports commit 8ea753718b2d1a42e9ce7b8db9f5e4e1f330e827 from qemu
Now that we have a DEFINE_PC_MACHINE helper macro that just requires an
initialization function, it is trivial to convert them to register a QOM
machine class directly, instead of using QEMUMachine.
Backports commit 865906f7fdadd2732441ab158787f81f6a212bfe from qemu
