Fix error reporting when poll returns POLLHUP/POLLERR

Fix error reporting when poll returns POLLHUP/POLLERR

In the RPC client event loop code, if poll() returns only a POLLHUP
or POLLERR status, then we end up reporting a bogus error message:

  error: failed to connect to the hypervisor
  error: An error occurred, but the cause is unknown

We do actually report an error, but we virNetClientMarkClose method
has already captured the error status before we report it, so the
real error gets thrown away. The key fix is to report the error
before calling virNetClientMarkClose(). In changing this, we also
split out reporting of POLLHUP vs POLLERR to make any future bugs
easier to diagnose.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>