webadmin: Enable "shareable" for preallocated floating disk.

"Shareable" option is valid for preallocated disk format and not valid
for sparse. Currently, this option becomes grayed out in UI while choosing
"sparse" disk format, the problem is that it stays grayed out even after choosing
"preallocated" format as described in BZ993148. This patch resolves that
issue.

Change-Id: I95c58cd001cfdf2f0d1a4fc75098b650f0c35fd3
Bug-Url: https://bugzilla.redhat.com/993148
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>