frontend: Creating VM without active storage domain

In past the engine allowed user to create a VM even if the SD was not
initialized (by manually selecting specific DC in the tree menu). This
patch brings this functionality back.

Change-Id: Ib1ae0b7564be7fa7072af6ab2a545affefbd0bed
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>