memory: fix off-by-one error in memory_region_notify_one()

This patch fixes an off-by-one error that could lead to the
notifyee to receive notifications for ranges it is not
registered to.

The bug has been spotted by code review.

Fixes: bd2bfa4c52e5 ("memory: introduce memory_region_notify_one()")
Cc: qemu-stable@nongnu.org
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Message-Id: <20171010094247.10173-4-maxime.coquelin@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
