core: Do not convert to preallocated images import to a block domain

Images that are imported to a block domain were converted to
preallocated since block domain does not support a raw-sparse image.
Instead now we convert the volume format from raw to cow only in case
that the original volume is raw-sparse

Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1130246
Change-Id: I519cf519857928899bf4cb4b65a862696e3a8aaa