memory: info mtree check mr range overflow

The address of memory regions might overflow when something wrong
happened, like reported in:

https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg02043.html

For easier debugging, let's try to detect it.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1489496187-624-1-git-send-email-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
