librdmacm: Fail ucma_init if ibv_get_device_list is empty

librdmacm: Fail ucma_init if ibv_get_device_list is empty

From the ibv_get_device_list man page:

   ibv_get_device_list() returns the array of available RDMA devices, or
   sets errno and returns NULL if the request fails. If no devices are
   found then num_devices is set to 0, and non-NULL is returned.

The librdmacm handles the failure case, but not the case where no
devices are found.  Handle that case as well.

Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>