KVM: SEV: initialize regions_list of a mirror VM

This was broken before the introduction of KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM,
but technically harmless because the region list was unused for a mirror
VM.  However, it is untidy and it now causes a NULL pointer access when
attempting to move the encryption context of a mirror VM.

Fixes: 54526d1fd5 ("KVM: x86: Support KVM VMs sharing SEV context")
Message-Id: <20211123005036.2954379-7-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/sev.c

@@ -2007,6 +2007,7 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
 	mirror_sev->fd = source_sev.fd;
 	mirror_sev->es_active = source_sev.es_active;
 	mirror_sev->handle = source_sev.handle;
+	INIT_LIST_HEAD(&mirror_sev->regions_list);
 	/*
 	 * Do not copy ap_jump_table. Since the mirror does not share the same
 	 * KVM contexts as the original, and they may have different