dump: add Windows dump format to dump-guest-memory

This patch adds Windows crashdumping feature. Now QEMU can produce ELF-dump
containing Windows crashdump header, which can help to convert to a valid
WinDbg-understandable crashdump file, or immediately create such file.
The crashdump will be obtained by joining physical memory dump and 8K header
exposed through vmcoreinfo/fw_cfg device by guest driver at BSOD time. Option
'-w' was added to dump-guest-memory command. At the moment, only x64
configuration is supported.
Suitable driver can be found at
https://github.com/virtio-win/kvm-guest-drivers-windows/tree/master/fwcfg64



Signed-off-by: Viktor Prutyanov <viktor.prutyanov@virtuozzo.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180517162342.4330-2-viktor.prutyanov@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>