KVM: ensure that memslot userspace addresses are page-aligned

Bad page translation and silent guest failure ensue if the userspace address is
not page-aligned.  I hit this problem using large (host) pages with qemu,
because qemu currently has a hardcoded 4096-byte alignment for guest memory
allocations.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>