KVM: Use kvm_kvfree() to free memory allocated by kvm_kvzalloc()

The following commit did not care about the error handling path:

  commit c1a7b32a14138f908df52d7c53b5ce3415ec6b50
  KVM: Avoid wasting pages for small lpage_info arrays

If memory allocation fails, vfree() will be called with the address
returned by kzalloc().  This patch fixes this issue.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>