librdmacm: Only allocate verbs resources when needed

The librdmacm allocates a PD per device on initialization.  Although
we need to maintain the device list while the library is loaded
(see rdma_get_devices), we can reduce the overhead by only allocating
verbs resources when they are needed.

This allows the rsocket preload library to support fork for
applications that spawn connections off to child processes.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>