x86, vmi: Disable highmem PTE allocation even when CONFIG_HIGHPTE=y

Preventing HIGHPTE allocations under VMI will allow us to remove the
kmap_atomic_pte paravirt op.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
LKML-Reference: <1267204562-11844-2-git-send-email-ian.campbell@citrix.com>
Acked-by: Alok Kataria <akataria@vmware.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>