CA-46110 Tab completion on xe-switch-network-backend

Moved xe-switch-network-backend from dom0.hg/overlay.dom0/opt/xensource/bin/xe-switch-network-backend to xen-api/scripts/xe-switch-network-backend. Added xe-switch-network-backend tab completion (CA-46110) to xen-api/scripts/xe-switch-network-backend-bash-completion. Updated scripts/OMake & xapi.spec.in file to accommodate changes.

* The script will still need to be removed from the dom0.hg but the build did not break. *