s390x/kvm: indicate alignment in legacy_s390_alloc()

Let's do this for completeness reason, although we don't support e.g.
PCDIMM/NVDIMM, which would use the alignment for placing the memory
region in guest physical memory. But maybe someday we would want to
support something like this - then we don't forget about this if
allowing multiple allocations in legacy_s390_alloc().

Use the same alignment as we would set in qemu_anon_ram_alloc(). Our
fixed address satisfies this alignment (1MB). This implicitly sets the
alignment of the underlying memory region.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180628113817.30814-3-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>