Fix qemu-nbd disconnect parameter
Fix qemu-nbd disconnect parameter

The parameter is wrong, it should really be /dev/nbd0.

Change-Id: Ifdb858cfa0e4116ca16fbc9c69a9f4f21f872164
Closes-Bug: #1305903

potsmaster/openstack-manuals,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals,gongysh2004/openstack-manual,gongysh2004/openstack-manual