KVM: Disallow fork() and similar games when using a VM

We don't want the meaning of guest userspace changing under our feet.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
index 1fd8158..be18620 100644
--- a/drivers/kvm/kvm.h
+++ b/drivers/kvm/kvm.h
@@ -305,6 +305,7 @@
 
 struct kvm {
 	struct mutex lock; /* protects everything except vcpus */
+	struct mm_struct *mm; /* userspace tied to this vm */
 	int naliases;
 	struct kvm_mem_alias aliases[KVM_ALIAS_SLOTS];
 	int nmemslots;