KVM: Don't null dereference ops->destroy

In the Linux kernel, the following vulnerability has been resolved:

KVM: Don't null dereference ops->destroy

A KVM device cleanup happens in either of two callbacks:

1) destroy() which is called when the VM is being destroyed;

2) release() which is called when a device fd is closed.

Most KVM devices use 1) but Book3s's interrupt controller KVM devices

(XICS, XIVE, XIVE-native) use 2) as they need to close and reopen during

the machine execution. The error handling in kvm_ioctl_create_device()

assumes destroy() is always defined which leads to NULL dereference as

discovered by Syzkaller.

This adds a checks for destroy!=NULL and adds a missing release().

This is not changing kvm_destroy_devices() as devices with defined

release() should have been removed from the KVM devices list by then.