scripts/dump-guest-memory.py: Introduce multi-arch support

By modelling the ELF with ctypes we not only gain full python 3
support but can also create dumps for different architectures more easily.

Tested-by: Andrew Jones <drjones@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1453464520-3882-6-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>