x86: don't ignore foreigndom input on various MMUEXT ops

x86: don't ignore foreigndom input on various MMUEXT ops

Instead properly fail requests that shouldn't be issued on foreign
domains or - for MMUEXT_{CLEAR,COPY}_PAGE - extend the existing
operation to work that way.

In the course of doing this the need to always clear "okay" even when
wanting an error code other than -EINVAL became unwieldy, so the
respective logic is being adjusted at once, together with a little
other related cleanup.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>