core: Set image format correctly when creating template image on block domain

When creating a template from a VM with a disk on an NFS domain without
snapshots (Raw+Sparse) on a block domain, the copy operation will fail
since this image configuration is not supported on block domains, this
patch causes the format to be changed from raw to cow in this situations
to avoid that problem

Change-Id: Icfa0843d3cad69682f6030ed47ec1699c106447a
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1131294