Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Make "options XENHVM" compile for i386, not just amd64 -- a largely
mechanical change. This opens the door for using PV device drivers
under Xen HVM on i386, as well as more general harmonisation of i386
and amd64 Xen support in FreeBSD.

Reviewed by: cperciva
MFC after: 3 weeks

Rename HYPERVISOR_multicall (which performs the multicall hypercall) to
_HYPERVISOR_multicall, and create a new HYPERVISOR_multicall function which
invokes _HYPERVISOR_multicall and checks that the individual hypercalls all
succeeded.

Import xen sub-arch includes.

MFC after: 2 weeks