qemu: Get capabilities to use iscsi password-secret argument
    
    Detect the capability via the query-qmp-schema for blockdev-add
    to find the 'password-secret' parameter that will allow the iSCSI
    code to use the master secret object to encrypt the secret for an
    and only need to provide the object id of the secret on the command
    line thus obsfuscating the passphrase.