vm-assist: prepare for discontiguous used bit numbers

vm-assist: prepare for discontiguous used bit numbers

Since the a flag will get assigned a value discontiguous to the
existing ones (in order to preserve the low bits, as only those are
currently accessible to 32-bit guests), this requires a little bit of
rework of the VM assist code in general: An architecture specific
VM_ASSIST_VALID definition gets introduced (with an optional compat
mode counterpart), and compilation of the respective code becomes
conditional upon this being defined (ARM doesn't wire these up and
hence doesn't need that code).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>