KVM: x86: switch kvm_set_memory_alias to SRCU update

Using a similar two-step procedure as for memslots.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -368,8 +368,12 @@ struct kvm_mem_alias {
 	gfn_t base_gfn;
 	unsigned long npages;
 	gfn_t target_gfn;
+#define KVM_ALIAS_INVALID     1UL
+	unsigned long flags;
 };
 
+#define KVM_ARCH_HAS_UNALIAS_INSTANTIATION
+
 struct kvm_mem_aliases {
 	struct kvm_mem_alias aliases[KVM_ALIAS_SLOTS];
 	int naliases;