bhyve: Make the MADT dynamically sized.

Use basl_ncpu instead of VM_MAXCPU in MADT_SIZE. Since several of the
offsets are no longer compile time constants, unroll the loop
generating ACPI tables.

Reviewed by: grehan
Differential Revision: https://reviews.freebsd.org/D34490

(cherry picked from commit 340a293f9147a37bb71c80a767e37794ca277a28)