iscsi: add support for getting CHAP password via QCryptoSecret API

The iSCSI driver currently accepts the CHAP password in plain text
as a block driver property. This change adds a new "password-secret"
property that accepts the ID of a QCryptoSecret instance.

  $QEMU \
     -object secret,id=sec0,filename=/home/berrange/example.pw \
     -drive driver=iscsi,url=iscsi://example.com/target-foo/lun1,\
            user=dan,password-secret=sec0

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1453385961-10718-4-git-send-email-berrange@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
