webadmin: Consolidated move & copy disk availability checks

The logic for checking whether moving a disk or copying a disk is allowed
was all the same except for the check whether the disk is used as a
template disk or not, both checks were consolidated to a single check
to avoid redundant checks on the same disks and for better code reuse

Change-Id: Ieb3c388aecc5b007a76c54ed0d41105bc28ebb51
Signed-off-by: Tal Nisan <tnisan@redhat.com>