kvm: Make VM ioctl do valloc for some archs

The kvm struct has been bloating. For example, it's tens of kilo-bytes
for x86, which turns out to be a large amount of memory to allocate
contiguously via kzalloc. Thus, this patch does the following:
1. Uses architecture-specific routines to allocate the kvm struct via
   vzalloc for x86.
2. Switches arm to __KVM_HAVE_ARCH_VM_ALLOC so that it can use vzalloc
   when has_vhe() is true.

Other architectures continue to default to kalloc, as they have a
dependency on kalloc or have a small-enough struct kvm.

Signed-off-by: Marc Orr <marcorr@google.com>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index de21d5c..d9305f1 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1852,13 +1852,13 @@ static void __unregister_enc_region_locked(struct kvm *kvm,
 
 static struct kvm *svm_vm_alloc(void)
 {
-	struct kvm_svm *kvm_svm = kzalloc(sizeof(struct kvm_svm), GFP_KERNEL);
+	struct kvm_svm *kvm_svm = vzalloc(sizeof(struct kvm_svm));
 	return &kvm_svm->kvm;
 }
 
 static void svm_vm_free(struct kvm *kvm)
 {
-	kfree(to_kvm_svm(kvm));
+	vfree(to_kvm_svm(kvm));
 }
 
 static void sev_vm_destroy(struct kvm *kvm)