prov/hook/trace: Add log on FI_VAR_UNEXP_MSG_CNT when enabled.

When using libfabric built with profiling interface enabled, the trace
hook can log the FI_VAR_UNEXP_MSG_CNT value for the changes on the
provider's unexpected message queue length if the provider supports
this feature.
This is an example on using profiling interface to read provider's varialbles.

Signed-off-by: Peinan Zhang <[email protected]>