scripts/dump-guest-memory.py: Move constants to the top

The constants bloated the class definition and were therefore moved to
the top.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1453464520-3882-2-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>