KVM: Make KVM_PRIVATE_MEM_SLOTS optional

Seems like everyone copied x86 and defined 4 private memory slots
that never actually get used.  Even x86 only uses 3 of the 4.  These
aren't exposed so there's no need to add padding.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

include/linux/kvm_host.h

@@ -313,6 +313,10 @@ struct kvm_irq_routing_table {};
 
 #endif
 
+#ifndef KVM_PRIVATE_MEM_SLOTS
+#define KVM_PRIVATE_MEM_SLOTS 0
+#endif
+
 #ifndef KVM_MEM_SLOTS_NUM
 #define KVM_MEM_SLOTS_NUM (KVM_USER_MEM_SLOTS + KVM_PRIVATE_MEM_SLOTS)
 #endif