Update usnic_devinfo for libfabric 1.4

[jsquyres]

Per libfabric v1.4.0 fi_ext_usnic(7):

o When FI_VERSION(1,4) or higher is used:
   o fabric name is the network address with the  CIDR  notation  (i.e., a.b.c.d/e)
   o domain name is the usNIC Linux interface name (i.e., usnic_X)

o When a lower version number is used, like FI_VERSION(1, 3), it follows
     the same behavior the usNIC provider exhibited in  libfabric  <= v1.3:
   o fabric name is the usNIC Linux interface name (i.e., usnic_X)
   o domain name is NULL