core: CDA to block disk alignment scan on file SDs

The current implementation of disk alignment scanning is useless for
file domains, and fails on NFS storage in particular.

This patch blocks such operations by a canDoAction(), to prevent any
useless calls to VDSM.

Change-Id: Ib7a9505ac16c9dd3cf14b584b8ec7e7292d68e98
Bug-Url: https://bugzilla.redhat.com/1046031
Signed-off-by: Allon Mureinik <amureini@redhat.com>