Only install the fake usnic libibverbs driver when there are actually
usnic kernel devices present. This prevents some run-time weirdness
on the Cray verbs emulation environment, where apparently
ibv_register_driver() either is not implemented or does not work
properly.
libibverbs will complain to stderr if it sees device entries in
/sys/class/infiniband for which it has no userspace plugins.
The Cisco usNIC device no longer exports a verbs interface, thereby
causing libibverbs to emit this annoying stderr warning.
To avoid this, use the public ibv API to register a "fake" libibverbs
driver at run-time (right after we call ibv_fork_init(), but --
critically -- *before* we call ibv_get_device_list()). The purpose of
this driver is solely to convince libibverbs that there *is* a driver
for /sys/class/infininband/usnic_verbs devices. ...although this
driver will never return a valid ibv context (and therefore will never
be used).
