webadmin: warning on move disks from file to block

Moving raw/thin disks that reside on a file domain
to a block domain will cause them to become preallocated.
Hence, it may consume considerably more space on
the target domain. To warn the user, added an
appropriate message to the move disks dialog.

Screenshot - http://i.imgur.com/gyMwXy9.jpg

Change-Id: I08fb316c76a9132467d26bbc5305a30a9883d38e
Bug-Url: https://bugzilla.redhat.com/1099083
Signed-off-by: Daniel Erez <derez@redhat.com>