block: add ability to set a prefix for opt names

When integrating the crypto support with qcow/qcow2, we don't
want to use the bare LUKS option names "hash-alg", "key-secret",
etc. We need to namespace them to match the nested QAPI schema.

e.g. "encrypt.hash-alg", "encrypt.key-secret"

so that they don't clash with any general qcow options at a later
date.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170623162419.26068-3-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>