Introduce kvm_get_max_memslots() API that can be used to obtain the
maximum number of memslots supported by KVM.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>