scripts/dump-guest-memory.py: fix int128_get64 on recent gcc

The Int128 is no longer a struct, reaching a python exception:
Python Exception <class 'gdb.error'> Attempt to extract a component of a value that is not a (null).:

Replace struct access with a cast to uint64[] instead.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=1427466



Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170310112819.16760-1-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>