volume: unify cow format check on creation

The same check performed both in fileVolume and blockVolume has been
moved to the base volume class. Now validateCreateVolumeParams in
fileVolume doesn't contain anything specific and it has been removed.

Change-Id: I78401df47278cdae34d824379acd0f2d088fa7b1
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28126
Reviewed-by: Dan Kenigsberg <danken@redhat.com>