scripts/dump-guest-memory.py: fix after RAMBlock change

commit 9b8424d5
    "exec: split length -> used_length/max_length"
changed field names in struct RAMBlock

It turns out that scripts/dump-guest-memory.py was
poking at this field, update it accordingly.

Cc: qemu-stable@nongnu.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1440666378-3152-1-git-send-email-mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 0c71d41e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>