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.

PR: 212782
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>
Approved by: bde (mentor), jhb (maintainer)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D18815