Move Open Firmware device root on PowerPC, ARM, and MIPS systems to
a sub-node of nexus (ofwbus) rather than direct attach under nexus. This
fixes FDT on x86 and will make coexistence with ACPI on ARM systems easier.
SPARC is unchanged.

Reviewed by:	imp, ian
Use the DELTA() macro to tidy the server-side interval stats code a bit.

MFC after:	2 weeks