KVM: s390: Remove the sanity checks for kvm memory slot

To model the standby memory with memory_region_add_subregion
and friends, the guest would have one or more regions of ram.
Remove the check allowing only one memory slot and the check
requiring the real address of memory slot starts at zero.

Signed-off-by: Nick Wang <jfwang@us.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>