KVM: selftests: Align memory region addresses to 1M on s390x

On s390x, there is a constraint that memory regions have to be aligned
to 1M (or running the VM will fail). Introduce a new "alignment" variable
in the vm_userspace_mem_region_add() function which now can be used for
both, huge page and s390x alignment requirements.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190523164309.13345-6-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
[prepare for THP as outlined by Andrew Jones]