KVM: s390: Add alignment checks for hugepages

KVM: s390: Add alignment checks for hugepages

When backing a guest page with a large page, check that the alignment
of the guest page matches the alignment of the host physical page
backing it within the large page.

Also check that the memslot is large enough to fit the large page.

Those checks are currently not needed, because memslots are guaranteed
to be 1m-aligned, but this will change.

Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>