arch/x86: warn if to-be-removed mapping does not exist

arch/x86: warn if to-be-removed mapping does not exist

Currently, when a memory mapping is removed with the memory_mapping
DOMCTL, no check is performed on the existence of such a mapping.
This commit adds such a consistency check to the code performing the
unmap, emitting a warning message if the check fails.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Cc: Dario Faggioli <dario.faggioli@citrix.com>
Cc: Paolo Valente <paolo.valente@unimore.it>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Julien Grall <julien.grall@citrix.com>
Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Eric Trudeau <etrudeau@broadcom.com>
Cc: Viktor Kleinik <viktor.kleinik@globallogic.com>
Cc: Andrii Tseglytskyi <andrii.tseglytskyi@globallogic.com>