virttest.iscsi: Only search luns under created target id

While decide lun id under new created target, other luns under
other targets need not be counted in, so only find the luns
under new created target string.

This update will help maintain the lun id as 1 under new created
target, which will avoid mismatch lun id when multiple luns
alreay exist on server.

Signed-off-by: Wayne Sun <gsun@redhat.com>