Don't allow MPtable generation if there are multiple PCI hierarchies. This is
because there isn't a standard way to relay this information to the guest OS.

Add a command line option "-Y" to bhyve(8) to inhibit MPtable generation.

If the virtual machine is using PCI devices on buses other than 0 then it can
still use ACPI tables to convey this information to the guest.

Discussed with:	grehan@