qcrypto/luks: implement encryption key management

Next few patches will expose that functionality to the user.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200608094030.670121-3-mlevitsk@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>