core: Disallow domains of mixed subtype (file/block) in <3.4 storage pool

Disallow adding domains of a different subtype (file/block)  that the
domains alread contained in the pool with a compatibility version lower
than 3.4 which contains domain

Change-Id: I8226c29e6c34ab6434d0a5ec07aa7fa1d4d00306
Bug-Url: https://bugzilla.redhat.com/1083560
Signed-off-by: Tal Nisan <tnisan@redhat.com>