engine,frontend: disable new VM/pool if no DC is UP

If there is no UP DC the new VM/Pool dialog will be disabled
and a message explaining why will be added.

Change-Id: Id537ac56bd2d963999c5c77ea3e92b3987d82525
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=990059
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>