qemu: Use secret objects to pass iSCSI passwords

qemu: Use secret objects to pass iSCSI passwords

https://bugzilla.redhat.com/show_bug.cgi?id=1425757

The blockdev-add code provides a mechanism to sanely provide user
and password-secret arguments for iscsi without placing them on the
command line to be viewable by a 'ps -ef' type command or needing
to create separate -iscsi devices for each disk/volume found.

So modify the iSCSI command line building to check for the presence
of the capability in order properly setup and use the domain master
secret object to encrypt the password in a secret object and alter
the parameters for the command line to utilize.

Modify the xml2argvtest to exhibit the syntax for both disk and
hostdev configurations.