MFC: r346715: Acpi MADT table correction for VM_MAXCPU > 21
The bhyve acpi MADT table was given a static space of 256 (0x100) bytes,
this is enough space to allow VM_MAXCPU to be 21, this patch changes that
so VM_MAXCPU can be of arbitrary value and not overflow the space by
actually calculating the space needed for the table.