webadmin: custom snapshot preview - disable illegal disks

Custom snapshot preview dialog:
* Disable selection for illegal disks ('N/A' instead of a check-box).
* Show relevant tool-tip on hover.

Change-Id: Ia4f7b7fd78437d5344852f22c85a3d0e11a6c6eb
Bug-Url: https://bugzilla.redhat.com/1111984
Signed-off-by: Daniel Erez <derez@redhat.com>