webadmin: create snapshot - filter out non-snapable disks

Create snapshot dialog:
Filter out non-snapable disks (shareable/DirectLUN) from list
(a relevant validation and warning already exists).

Change-Id: I7bfc4fbaa254ca7ac8c5f37decbcd78fcf991636
Signed-off-by: Daniel Erez <derez@redhat.com>