kern/148262: [xen] [panic] HYPERVISOR_update_va_mapping

[Juergen Lock]

[kern/149853 is a duplicate of this]

I don't use xen myself here but I helped someone on irc troubleshoot
the same crash (before I saw these PRs), and found out the reason it
didn't happen on 8.0 is the x86bios code is new in 8.1 and xen doesn't
like this call in x86bios_map_mem():

	x86bios_ivt = pmap_mapbios(X86BIOS_IVT_BASE, X86BIOS_IVT_SIZE);

see:

	http://fxr.watson.org/fxr/source/compat/x86bios/x86bios.c?v=FREEBSD8#L498

It seems the only consumers of x86bios are atkbd(4), dpms(4), and
vesa(4), and indeed removing atkbd(4) from the xen kernel got his
domU running.

(And now that I just looked at /sys/i386/conf/XEN in svn I see this
was already fixed in r204157, it only never got MFCd.)