fabrics: lookup_discovery_ctrl() must look under host and not root

fabrics: lookup_discovery_ctrl() must look under host and not root

lookup_discovery_ctrl() was scanning the whole root looking for any
matching discovery controller (DC). Instead, it must look under the
host object. The problem happens when there are more than one Host
NQN used to connect to the same DC. In this case, we will have more
than 1 host object and it is important to search for existing DCs
under the right host object.

Signed-off-by: Martin Belanger <martin.belanger@dell.com>