dump-guest-memory QMP command: check vmcore correctness

Before upstream qemu commit

  commit 56c4bfb3f07f3107894c00281276aea4f5e8834d
  Author: Laszlo Ersek <lersek@redhat.com>
  Date:   Tue Aug 6 12:37:11 2013 +0200

      dump: rebase from host-private RAMBlock offsets to guest-physical
      addresses

the dump-guest-memory QMP command used to write incorrect vmcores for
x86_64 guests with more than 3.5GB RAM. Add a testcase that creates a 4GB
guest, dumps its core, copies it into the guest, and analyzes it there
with the "crash" utility.

Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=981582
Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=990118

Suggested-by: Ademar de Souza Reis Jr. <areis@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>