Pass CONFIG_FUZZ via host_kconfig, and use it to select the
sparse-mem device.

Cc: Alexander Oleinik <alxndr@bu.edu>
Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20211007130829.632254-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>