block: pass option prefix down to crypto layer

block: pass option prefix down to crypto layer

While the crypto layer uses a fixed option name "key-secret",
the upper block layer may have a prefix on the options. e.g.
"encrypt.key-secret", in order to avoid clashes between crypto
option names & other block option names. To ensure the crypto
layer can report accurate error messages, we must tell it what
option name prefix was used.

Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Daniel P. Berrange <[email protected]>
Message-id: 20170623162419[email protected]
Signed-off-by: Max Reitz <[email protected]>