librdmacm: Support lazy initialization

librdmacm: Support lazy initialization

librdmacm currently opens a device context per configured HCA. This is
usually done in rdma_create_event_channel() or first time whenever
ucma_init() is called. If a process is only going to use one of the
configured HCAs/RDMA IPs then the remaining device contexts are not
used/required. Opening a device context on each device apriori limits the
maximum number of processes that can be supported on a node to the maximum
number of open context supported per HCA regardless of number of HCAs present
in the system.

Signed-off-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>