Makes it slightly less likely to forget a check and end up doing an
out-of-bounds access. Also makes it obvious that we *are* indeed
checking whether the handle is valid, instead of hiding it in
HasOwnership (which won't handle the root key handle case properly).
This prevents the IOS crypto code and keys from being spread over
the codebase. Things only have to be implemented once, and can be
used everywhere from the IOS code.
Additionally, since ES exposes some IOSC calls directly (DeleteObject
and Encrypt/Decrypt), we need this for proper emulation.
Currently, this only supports AES key objects.