iso element to use 'search --label' for grub

Current iso element uses 'search --file' to find the
grub root.  This is a wrong approach as it may find
some other partition containing /vmlinuz. Instead modify
it to search for grub root by the label of ISO image.

Closes-Bug: 1384826
Change-Id: Id4217be3420597bed2f80a96788928259ec91582