webadmin: Block adding of mixed domain to a pool if not supported by version

When adding a new storage domain via webadmin where the compatibility
version does not support mixed storage types we might encounter a situation
where the domain is created and the attach to the data center is failing
and the setup is left with an unattached domain, to prevent that we now
disallow addition from a storage type (block/file) to a data center that
does not support mixed types and already have attached domains of another
storage type (e.g. adding an iSCSI storage domain to a v3.3 data center
V3.3 with an NFS domain attached will be prohibited)

Change-Id: Ib31884e6fd5a7868e9bb21c9566bcd2e445de52a
Bug-Url: https://bugzilla.redhat.com/1085395
Signed-off-by: Tal Nisan <tnisan@redhat.com>