prov/verbs: Fix CQ read call to return correct error code

If poll CQ returns a work completion with a failed status,
the CQ read calls will return an incorrect value.  Return
FI_EAVAIL as documented.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>
Merge pull request #376 from shefty/master

prov/verbs: Fix CQ read call to return correct error code