Update description of Enable block_migrate_cinder_iscsi

This option, originally added in
I81cc39fed392fce9dfbda5683ffba29eeba936be was disabled by default for
Xen, and isn't supported by libvirt due to bug 1398999. Provide a better
help message explaining why this should be off for libvirt.

Change-Id: Id8f9ead99b5d5713172c9b012b771098fdee7b1c
Depends-On: I89b7e390bf1cf4f2eccabca2e31a9d1b6b270677
2 files changed