use image path only once in manifest file

image path for each image should appear only once in the manifest file
even if `zvsh` was provided with multiple `--zvm-image=` options for the same file
fixes issue #50
better handle multiple fstab entries

`nvram_fstab` was using `dict()` which does not provide mutiple entries for the same device
it's needed when the same file is mounted with different mount points or access flags
Merge pull request #51 from pkit/bug_fix

use image path only once in manifest file