arch/arm: unmap partially-mapped memory regions

arch/arm: unmap partially-mapped memory regions

This commit modifies the function apply_p2m_changes() so that it
destroys changes performed while mapping a memory region, if errors are
seen during the operation. The implemented behaviour includes destroying
only mappings created during the latest invocation of apply_p2m_changes().
This is useful to avoid that memory areas remain partially accessible
to guests.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.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: Jan Beulich <jbeulich@suse.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>
Acked-by: Ian Campbell <ian.campbell@citrix.com>