s390x: Enable KVM huge page backing support

s390x: Enable KVM huge page backing support

QEMU has had huge page support for a longer time already, but KVM
memory management under s390x needed some changes to work with huge
backings.

Now that we have support, let's enable it if requested and
available. Otherwise we now properly tell the user if there is no
support and back out instead of failing to run the VM later on.

Signed-off-by: Janosch Frank <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Message-Id: <20180802070201[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>