Acceptance tests: expect boot to extract 2GiB+ initrd with linux-v4.16

XLF_CAN_BE_LOADED_ABOVE_4G is set on vmlinuz shipped by Fedora-28 so that
it's allowed to be loaded below 4 GB address.

timeout is updated to 5 minutes as well since we need more time to load a
large initrd to the guest

CC: Wainer dos Santos Moschetta <wainersm@redhat.com>
CC: Caio Carrara <ccarrara@redhat.com>
CC: Cleber Rosa <crosa@redhat.com>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <1548638112-31101-2-git-send-email-lizhijian@cn.fujitsu.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>