KVM: explain missing kvm_put_kvm in case of failure

The call to kvm_put_kvm was removed from error handling in commit
506cfba9

 ("KVM: don't use anon_inode_getfd() before possible
failures"), but it is _not_ a memory leak.  Reuse Al's explanation
to avoid that someone else makes the same mistake.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>