Acceptance tests: use linux-3.6 and set vm memory to 4GiB

QEMU have already supported to load up to 4G initrd if the sepcified memory is
enough and XLF_CAN_BE_LOADED_ABOVE_4G is set by guest kernel

linux-3.6 kernel shipped by Fedora-18 cannot support xldflags so that it
cannot support loading more than 2GiB initrd

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: <1548638112-31101-1-git-send-email-lizhijian@cn.fujitsu.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>