KVM: x86: Allocate memslot resources during prepare_memory_region()

Allocate the various metadata structures associated with a new memslot
during kvm_arch_prepare_memory_region(), which paves the way for
removing kvm_arch_create_memslot() altogether.  Moving x86's memory
allocation only changes the order of kernel memory allocations between
x86 and common KVM code.
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -9875,6 +9875,12 @@ void kvm_arch_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free,
 
 int kvm_arch_create_memslot(struct kvm *kvm, struct kvm_memory_slot *slot,
 			    unsigned long npages)
+{
+	return 0;
+}
+
+static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
+				      unsigned long npages)
 {
 	int i;
 
@@ -9966,10 +9972,9 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 				const struct kvm_userspace_memory_region *mem,
 				enum kvm_mr_change change)
 {
-	if (change == KVM_MR_MOVE)
-		return kvm_arch_create_memslot(kvm, memslot,
-					       mem->memory_size >> PAGE_SHIFT);
-
+	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
+		return kvm_alloc_memslot_metadata(memslot,
+						  mem->memory_size >> PAGE_SHIFT);
 	return 0;
 }