memory: fix 128 arithmetic in info mtree

mtree_print_mr() calls int128_get64() in 3 places but only 2 places
handle 2^64 correctly.

This fixes the third call of int128_get64().

Cc: qemu-stable@nongnu.org
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>