Add support in GUI to specify LUN ID for iSCSI Target->Extent

This is required for the VMWare certification.

(cherry picked from commit 3f0e766fd2aaaf91317443962095c3cb9e61201f)
LUN ID can start with 0

(cherry picked from commit 2083d29f7a5150cd467318e3425a6cef93dd1d5d)
Backend for the new iSCSI Target->Extent LUN ID

(cherry picked from commit e4f3dea2dd3129017e0422faa01f3b095ca86e40)